1969 Chevrolet Chevelle Loud clackity/bang sound

DAYDREAMINGYPSY

MEMBER
1969 CHEVROLET CHEVELLE
0.9L
V8
RWD
AUTOMATIC
200,000 MILES

Hello, I have a 69 Chevelle Malibu with the original 307 engine. It had headers on it that my dad had put on quite a few years ago. They had a few holes in them and were pretty worn out. When I replaced them I chose to put stock exhaust manifolds back on. Mostly due in part to easier accessibility to oil filter and spark plugs etc My mufflers were also pretty shot. A friend of mine gave me a set of glass packs to put on it. It sounds really good except for one thing. There's a loud "undercurrent" sound that is making me CRAZY! It's not really loud when I first start it but it gets louder as the engine warms up. It's kind of a clackety/bangy kind of noise. Almost sounds like a double head hammer being banged back and forth between two pieces of thick metal. I have double and triple checked the ALL the welds. They're good except for one small pin-hole on the drivers side, however i'm reasonably certain that that is not my problem. The flexplate was replaced about 2 yrs ago because the tranny bolts had somehow come loose and the tranny was slipping back and apparently caused the flexplate to crack. The guys at the shop said it came off in two pieces. Seeing as how they were the ones who noticed that the tranny bolts were loose, I am somewhat confident that they tightened it properly. So I don't think it's the flexplate again (i could be wrong, idk?) I know a couple "home mechanics" that I believe "know their stuff", however for some reason they can neither one hear it. I've used a stethoscope on the block and tried the old "cut a wooden broom handle off and hold one end to the engine block and the other end to your ear " trick (does that really work in your opinion?) And didn't hear any crunchy, clackity, bangy, grindy, thumpy or tickety sounds. It actually sounded surprisingly smooth for it's age and mileage. So next I used the stethoscope and went along the length of the pipes. I could definitely hear it but still couldn't tell exactly where it is coming from. The sound does NOT stop when I stop the car so i'm sure it's not something that's on/in the drive train. So I'm wondering, is that the way glass packs are supposed to sound? I've never had a pair so I don't know how they're supposed to sound. Do glass packs "go bad" inside and if so how can you tell? Thank you for your time and opinion! Vikki

3 Replies

HMAC300

MECHANIC
48,601 POSTS

Glass packs get noisier after a period of time but it's not a sound like you are hearing. From your description I'd check to see how close the exhaust system is to the frame parts kinda sounds like the pipes are hitting against it someplace like maybe a spring in rear. Glass packs really sound good when you back down in 2nd or third gear. And yes take tip from a 50 yr mechanic the broom handle thing works.

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Tuesday, March 18th, 2014 AT 6:21 AM

DAYDREAMINGYPSY

MEMBER
2 POSTS

Hi Hmac, Thank you for the reply to my question. Unfortunately nothing in my life is ever that simple or easy! Especially if it has tires or testicles (no offense meant!) I had the same thought, that the pipes might be hitting against something.I have a big (bumper?) Jack that I put under the bumper (of course) to lift it up and then I used a floor jack under the rear pumpkin and lifted it so that the axle was as close as I thought it was when the car is sitting on the ground.I could then put it in gear and watch to see if it looked like anything was hitting.I couldn't find a problem. Plus it bangs with the speed of the engine. It does seem louder on colder days.I thought at first it might be a rod knock but like I said, when I used my stethoscope and the broomhandle on the engine block everything sounded like it was running surprisingly smoothly. Would I be able to hear a rod knocking with the broomhandle? It didn't make this sound when it had the headers on it even with the holes in them. There was a slight "ticking" sound that's typical with an exhaust leak but this is much louder. Maybe I just worry too much and it's really nothing, but it's making me CRAZY! I'm even beginning to regret the exhaust exchange. Even considering putting my holey headers back on!(I can't afford new ones) :( Thanks again for answering and if you think of anything else it might be please let me know! Thanks again, Vikki

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Saturday, March 22nd, 2014 AT 5:12 PM

HMAC300

MECHANIC
48,601 POSTS

A rod knock you can hear with out the broom handle and you can short out the cylinder to eliminate it anyhow most of the time. That means pulling the plug wire off while running if the noise goes away it's either a piston problem or rod knock but you would have low oil pressure as well with the rod. One thing that these things used to do is the torque converter bolts would loosen and hit the back of block so see if any are loose/backed off. If so put Loctite on them(blue) and retighten and the noise should go away. Also checkfor a broken motor mount torque in drive with hood open if engine lifts high then it's broken do the same in reverse. Have your key ready to turn off when doing this as these have been known to fly away on ya.

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Sunday, March 23rd, 2014 AT 8:51 AM