Can anyone diagnose by car problem?

May 31, 2009 1:24 PM

What the heck is wrong with my car? And no, you are not my mechanic.

Alright so I have an automatic 2004 Subaru Legacy sedan. Just had some work done to it but a new problem just started to pop up. I should point out I first noticed this just before I had the work done, so it is not related. But if it helps I had some oil leaking onto one of the spark plugs causing hesitation.

I am asking in the hopes of softening the blow when I do have it looked at this week.

So when I am at a stop light or sign and I hit the accelerator hard there is a rumble/vibration right under my seats, maybe a little further back. It can be felt and heard. If I accelerate very slowly this does not happen. No problems turning wheel, no noises while turning so hoping it is not the differential. It only happens in the above situation. I do notice that when I am at highway speeds that there is an occasional rumble but much higher frequency.

Possibly related. There has been a rattling of what sounds like a heat shield when I am driving. I have looked under the car to find the culprit but don't see anything noticeably loose. The sound also seems to be coming from the center of the car, under the driver seat, center console. Think there is a cat converter under there.

So any suggestions? I have been looking online and the only possibility I came across was something to do with the drive shaft or drive shaft bearing if I remember correctly. Does that make sense?

I am really really really hoping it is not my transmission. Really. I don't notice any slipping when driving or other transmission red flags. For what it is worth.

posted by WickedPissah to travel & transportation (13 comments total)

Sounds like a constant velocity/universal joint going bad.
posted by torquemaniac at 1:27 PM on May 31

Motor mount.
posted by gjc at 1:30 PM on May 31

It sounds like you have already hit it - a loose heat shield between the exhaust and the car. It will be above the exhaust, most likely in the transmission tunnel and so not visible unless you get properly underneath it, and will have a couple of resonant points where it could make noise. This could explain the two types of noise.

Also, it could be a propshaft (I believe US'ians call it a driveshaft - between the gearbox and the diff, that travels the length of the car) bearing, but it's less likely in a car of that age. if it is a propshaft bearing, it may not be too expensive, unless they only sell it as a complete unit. It could be anything between $50-300 for parts and a few hours labour, I'd guess. It doesn't sound, though, like it's an internal transmission issue, to me.

Another option for the hard acceleration noise is a worn engine/drivetrain mounting bush, where the torque applied twists the transmission enough that it contacts the body (ie no longer isolated by the rubber mount) this makes a surprising amount of noise. Again, though, this shouldn't be hideously expensive to fix. The higher frequency rattling is still most likely a heat shield, although the mount may have contributed to this coming loose.
posted by Brockles at 1:32 PM on May 31 [1 favorite has favorites]

Subarus are notorious for loose heat shield problems. I've owned six and it eventually has happened to all of them. Usually the diagnosing factor is a rumbling sound, as noted above, it is not always visual.
posted by Xurando at 1:58 PM on May 31

Subaru exhaust sheilds are the worst. I've cut literally dozens of them off with tin snips due to customer complaints. You might not always be able to detect which one is loose by laying on the ground and tapping on the exhaust. You may have to put in on a lift and knock around with a mallet. When you find a bad one, you can snip it off or put a new band clamp or two on to tighten it down.

If you've kept up on your maintenance, 2004 would be really young to need a transmission so that's pretty unlikely.

A motor mount would manifest really dramatically when going from park to drive to reverse as a clunk or shudder. To test, find a safe place (empty parking lot), set your parking brake, put one foot on the brake (FIRMLY), put the car in drive and give it a couple revs. If you get a really pronounced clunk, you've got a bad motor mount. Sometimes, with the hood open, you can peek through the gap and see the motor flailing around.
A bad driveshaft or driveshaft bearing will usually make an intolerable vibration at speed and I haven't replaced any, that I can remember, on Subarus.
posted by Jon-o at 2:26 PM on May 31 [2 favorites has favorites]

Seconding Jon-o about the motor mounts. I had them go on my car years ago and it wasn't pretty. Started at an intersection after the light turned green and when I accelerated, the engine jumped ahead and the fan went with it into the rad. Ouch!

IMO your problem is likely a heat shield or something to do with the converter. Maybe the converter has two layers of metal covering it and one has rusted through or peeled away a bit.
posted by Taurid at 4:55 PM on May 31

Thanks all for the help so far, puts my mind at ease a bit.

Maybe I should be more clear, it is more the feeling rather than the sound that I am getting concerned about. The heat shield sound (assuming the are seperate) is a rattle sometimes a high pitched cricket chirp sound at speed.

The more disconcerting thing is that weird rumble feeling at acceleration. Thump-thump-thump right under the seat/center console. Goes away after three-four thumps. Could that be the mount? Guess it's in time with the acceleration so it is a quick thumping but nothing that I have ever felt on a car before. So I am baffled.
posted by WickedPissah at 8:09 PM on May 31

Could also be a bad transmission mount... but more likely as others have said the heat shield. I had a bad transmission mount in a Subaru and only noticed it turning right while going over a bump. Got a clunk-clunk and could feel it but really had a hard time identifying where it was. Hard to identify as even though it seemed like it was in the front suspension it was actually from a rear transmission mount.
posted by cftarnas at 11:28 PM on May 31

You mentioned that the thump/rumble happens again on the highway at a higher frequency. Is there an accompanying vibration in the steering wheel or in your seat?
posted by Jon-o at 4:02 AM on June 1

Well it may not be related to the thump on acceleration. But I have been noticing that every now and then there will be more of a sound like I am driving on a rumble strip but not as jarring, just sort of a high frequency hum. This only happens on the highway. And no shake really at all. Not in the wheel or the seat. It just comes and goes.

I did just try the hit the accelerator with my foot on the brake and could not duplicate the thumping.

I am hoping to drop it off early tomorow and hopefully I can have it taken care of. But if anyone else has any suggestions I am more than willing to listen to them.
posted by WickedPissah at 6:42 AM on June 1

A heat shield will make clink, think, pling kinds of sounds. It's just a hunk of aluminum hanging in the breeze. A high frequency hum, approaching a buzz, is probably going to be engine speed or wheel speed related. That's why I suggested motor mount- under heavy acceleration, the engine is flexing too much and making contact with the frame.

Could also be an exhaust leak that only exposes itself under certain situations. This would be a put put put kind of sound, or thwat thwat thwat.
posted by gjc at 7:11 AM on June 1

Your high-frequency hum could be an irregularly worn tire or a bad wheel bearing.
posted by Jon-o at 9:26 AM on June 1

Turns out it was the drive shaft. It was getting bound up and thumping the underside of the car. It was the cause of all three sounds heat shield sound, thumping and higher frequency hum on the highway. It was removed and rebuilt and reinstalled.
posted by WickedPissah at 4:18 PM on June 10