Knocking sound under front of vehicle

Tiny
MARTIH07
  • MEMBER
  • 2005 PONTIAC BONNEVILLE
  • 78,000 MILES
We have had our 05 Pontiac Bonneville in three different shops looking to solve a thumping/knocking noise under the front end. We have had the intermediate shaft replaced, the shocks replaced, swapped out tires on the car and still, when driving over any bump or rough road, it sounds like something is loose and knocking. You can hear it and feel it in the steering wheel and in the gas pedal. Thump thump thump

The car was being driven about 1 1/2 years ago and the driver ran smack into a curb that was about 4-5 inches high. We've been thinking that perhaps something got knocked out of shape, or something loosened but none of the techs we've taken it to have been able to find anything wrong.

Any ideas of where to look next?

Thanks!
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Thursday, May 31st, 2012 AT 6:47 PM

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Tiny
CARADIODOC
  • EXPERT
There is a tool you might be able to borrow or rent from an auto parts store that borrows them called the "Chassis Ear". It is a set of six microphones, a switch box, and headphones. You clip the microphones to suspect points, then drive around while listening with the headphones. You can move the microphones around to zero in on the source of the noise. Be aware that many mechanics have never seen or even heard of this tool. Suspension and alignment mechanics use it to find rattles, squeaks, and other noises.
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Thursday, May 31st, 2012 AT 6:54 PM
Tiny
MARTIH07
  • MEMBER
Thanks Caradiococ! You know, they actually did that yesterday and. Nothing. I hadn't even heard of being able to do such a thing! I have it back at the dealer today. Maybe today they'll find something. Makes you feel like you're nuts, but the good thing is that they hear it too! They just can't find it!
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Thursday, May 31st, 2012 AT 8:41 PM
Tiny
CARADIODOC
  • EXPERT
There seems to be a lot more trouble on GM cars with lower control arm bushings than in years past, at least I read about them causing noises more than before. Another elusive problem is worn inner anti-sway bar bushings. On some models you can remove them and turn them around and they'll be quiet. The center holes wear at an angle and turning them around in effect changes the angle the bar goes through them and the resulting tension keeps the bar from rattling. That is not the proper fix but it is a way to confirm the cause before buying parts.

Another thumping noise that can be hard to find is worn front struts. If the shaft wobbles where it comes out of the top of the strut body, the thumping can transmit to the other side of the car through the steering linkage. To identify that, reach over the top of the tire, through the coil spring, and push up the hard plastic boot, then hold your fingertips lightly on top of the strut body with your fingertips just touching the shaft. Now use your arm to push up and down on the fender a little. Next, use your free hand to tug in and out on the top of the tire. You will feel the shaft moving up and down a little in the strut body, but what you're really looking for is any in and out movement of the shaft against your fingertip. Also, if there is any sign of oil on top of the strut body, it is leaking and needs to be replaced anyway so have that done, THEN continue looking for the noise if it's still there.

The upper strut mounts are known for causing a thumping noise when turning the steering wheel when they bind but that almost never happens while the car is moving. The bearings break apart or get dirt in them and prevent the strut from turning freely. Since they're holding up the weight of the car, any binding will prevent the top of the coil spring from rotating freely when turning the steering wheel. To identify that, reach over the tire and lightly wrap your fingertips around an upper part of the spring, then have a helper slowly turn the steering wheel left and right. The spring should rotate rather smoothly. If you feel it wind up, then suddenly pop free and turn, the upper mount is binding and needs to be replaced. The center hole where the strut shaft comes through can rust out too and let the shaft and wheel move left and right. That can cause a thumping noise but it's usually accompanied by a complaint of steering wander. That type of wear is often not noticed by the owner and is unknown until the struts are removed for normal maintenance replacement, THEN the mechanic has to find you and tell you more parts are needed. They hate doing that so many shops just prepare for that possibility and include new upper mounts in the repair estimate. You might be able to identify that problem by pushing and pulling really hard on the top of the tire while watching the larger nut in the center of the mount. They're in the middle of the black or silver plates under the rear corners of the hood, but on many cars there will be plastic trim covers that must be popped off to see them. You can also identify the mounts by three nuts around them holding each one to the inner fender.

The mounting bolts for the steering rack and pinion should be checked for tightness along with inspecting the tie rod ends. Something could be coming apart inside the rack assembly too although that usually results in fluid leakage.

You might also ask if anyone checked for service bulletins that might apply. Those are not the same things as recalls. Recalls cover safety, emission, and customer satisfaction problems that affect all cars of a certain model and year. Service bulletins are mainly for information on hard-to-find problems that might affect multiple cars of that model, and someone has already identified the cause and fix. They are meant to help other people solve the same problem without repeating all the same time-consuming diagnostics. There was one popular vehicle many years ago that could develop an "oil can" effect, (remember the old metal oil cans that you pressed on the bottom to get a squirt of oil out?). The firewall was flexing on rough roads more than normal. It had an easy fix but the cause was impossible to locate without that service bulletin.

Another trick is to look over the bottom of the car for any place the paint has been rubbed off, particularly on the firewall. Collapsed engine mounts can let the drive train hang down on one side and place engine accessories near splash shields and brackets.

Look at all of the exhaust system hangers. They will always be two pieces of metal separated by a piece of rubber. That rubber can be riveted to each metal part or a part of the bracket can be slipped through the center of the rubber "isolator". If any hanger is damaged or worn to allow the two metal parts to touch, the thumping and vibration will be transmitted into the entire car body. You will feel that in the gas pedal and steering column because both are bolted to the firewall.

Put the car in "park", pop the hood, then try to push the car backward and forward by hand. The engine will rock back and forth when you do that but it shouldn't make any noise and it should stop rocking smoothly, not with a sudden bang. If you hear metal-on-metal when the engine comes to a stop, or if it rolls really far, look for a "dog bone" metal bracket between the core support, (just above the radiator), and engine. Some GM cars have two. It's pretty common on GM front-wheel-drive cars for the bushings to deteriorate in that bracket and allow that metal-on-metal thumping. That typically occurs once when accelerating and once again when letting off the gas at higher speeds. There is a "problem solver" part made to replace that bracket that looks like a tiny shock absorber. On some cars, if the engine rocks forward enough, it can raise the rear exhaust manifold up to where the exhaust pipe hits the firewall.

This doesn't really match your description of the sound but with the hood closed, press down on the center along the front, then do that at each front corner. If you can move either corner down by hand, there is an adjustable stop that the hood isn't resting against. Unscrew it counter-clockwise to raise it up, then close the hood and check that the hood matches the fender. If it's adjusted too high, the hood will be higher than the fender. Those two stops provide part of the "pop" that raises the hood when you pull the release.

If you can press the center of the hood down by hand and feel any clunking, the striker is adjustable. Adjust it to hold the hood down tighter. Be careful when pressing on the hood because the sheet metal isn't what it used to be. It's real easy to leave small dents from your fingers.

If you have to use a stick to hold your hood open, look at the props to see if they are rattling, similar to checking the struts. If the oil has leaked out and they rattle, that can transmit to the firewall although it will typically feel lighter than your typical clunk. If the hood props are working even partially, the gas pressure will hold the shaft from rattling.

A really weird one you might consider is a rattling windshield although if that wasn't causing a problem before, it usually takes some type of body damage to make it occur. On some cars, plastic inserts are used on the assembly line to set the position of the glass and to hold it there until the urethane sets up. Later those clips are supposed to dissolve when they get wet. If the windshield is replaced and not positioned correctly or if the body is twisted in the right way, the edge of the glass can knock against the sheet metal frame. I've never heard that myself so I don't know what it sounds like, but there is a service bulletin on it. That urethane remains somewhat flexible but the glass is actually considered a structural element of the body so it's easy to understand that it will be under stress at times and will move a very little amount.

You might observe if you never hear the thumping when the wipers are running. If the noise only occurs when they are in the lowered "park" position, check the linkages to see if one is making contact with the body.

Try lifting the front bumper cover by hand. Squat in front of one corner, then use your knees to help push the bumper up. If you feel any movement or thumping, look at the large impact-absorbing shock absorbers it's mounted on to see if there's any damage or wear.

Another one that's really hard to find is a loose engine cradle mounting bolt. On many GM front-wheel-drive cars, the cradle must be removed to remove the engine or the transmission from below the car instead of out the top like most other cars. That cradle is also removable on Chrysler cars and minivans but they use special bolts to locate it correctly when it's reinstalled. Those bolts will hold it from rattling if one is accidentally left loose so you would never know. On GM cars, that cross member can be shifted around when all four of the bolts are loose and that will lead to severe misalignment of the wheels and a very miserable car to drive. That cradle must be located precisely, otherwise even an alignment will not resolve the terrible handling. To my knowledge, that only is a potential problem on GM cars. I'm just mentioning that so no one jumps in and loosens them all up to see what I'm talking about. If just one bolt is loose, the cradle can't really shift position but it could cause a thumping when the body flexes.

There's some things I've found or heard about in the past. Start by checking steering and suspension parts that attach to anything that bolts to the firewall since that's where you're feeling the thumping.
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Friday, June 1st, 2012 AT 4:45 AM

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