2002 Subaru Forester All Wheel Drive Automatic 165000 miles
I have a high pitch hum which sounds like its coming from the back. My mechanic said they can't find the source until it gets louder/worse. It starts at around 40 mph after I've been driving for a couple of blocks. Its louder at higher speeds and when I take my foot off the accelerator and coast. Seems to be getting worse. Has been there for about a week. Tires are less than a year old. A friend heard the noise and also thinks its coming from the rear of the car. Kind of sounds like that high pitch hum you hear from a semi truck. At first I thought it was from a bigger SUV next to me on the road. Thanks!
It is REAL easy for noises to be transmitted, so don't overlook things in the front. Front wheel bearings will sound like an airplane engine, and will often get louder or quieter when turning slightly such as when changing lanes. A rear bearing can do that too, but front ones are more common because of the drivetrain weight they have to hold up.
Look for a mechanic who has a " Chassis Ear". This is a set of six microphones you clip onto suspected noise sources, then you drive the car, listen with headphones, and switch between the microphones. By determining the loudest one and moving them around, you will zero in on the noise source.
March, 29, 2010 AT 4:28 PM
The noise gets louder when I go faster or when I take my foot off the accelerator. After I slow down it stops but starts again when I go faster. There's no difference in the noise while turning. And there's no noise at all when I'm stopped. Wondering if its a rear differential/gearing, bearings or other rotation/wear issue?
March, 29, 2010 AT 5:00 PM
All those things are possible, but you're going have to listen next to the source, either with microphones or by running it in gear on a hoist.
Not all wheel bearings change sound when turning. That was common on older cars with pressed-in bearings. Most cars now use bolt-on assemblies. Even when their sound changes, you can't be sure where the noise is comming from. The only way I found to tell for sure was to run it on a hoist and listen with a stethoscope. One will sound a little rumbly so you won't be sure, but the other one will be obvious it has a problem. You won't hear the noise by ear that way because there is no weight on them.
Gear noise is usually a pulsing howl. Here again, it might not do it on a hoist when there is no load on it.
Your best bet is the Chassis Ear. You might be able to rent one at one of the auto parts stores, but a lot of people won't even know what you're talking about. The guys who drive the tool trucks like Matco, Mac, and Snapon can order them. The old version has 6 microphones with wires that must be carefully run to avoid hot and moving parts. The newer version has two wired mics and four are wireless. The old version, as I recall, runs around $100.00.