October 20, 2006 3:48 PM   Subscribe

What's rattling under the hood of my car?

I have a 98 automatic Corolla. When it's in neutral, or stopped at a light/stop sign, there's a pretty loud rattling coming from under the hood. When I put it in park, the rattling stops. When I'm driving, no rattling. Perhaps related, I've noticed that the transmission has been kind of wonky and not as responsive as it once was -- that is, it doesn't shift into the gear that I feel like it should be in as quickly as it ought to, and sometimes feels like it's starting in second. That may be a separate issue, though, since the car has about 118,000 miles on it. Any thoughts on either of these issues is appreciated.
posted by one_bean to Travel & Transportation (6 answers total)
On the second, a filter and fluid change in the tranny might be something to consider.

On the first, it's probably a bracket or similar that is loose, assuming it's an external-sounding rattle and not an internal noise (to the engine, that is). Because the car idles at a lower engine speed in gear than it does in park, you're probably just getting the right frequency going to make something unhappy about how loose it is.

Get a friend to poke their head under the hood and see if they can find the source of the rattle when you have the car idling in gear. Don't run them over.
posted by maxwelton at 4:01 PM on October 20, 2006

Oh I forgot to mention that I got the transmission fluid changed at ~95,000 miles (though I don't know if they changed the filter). Thanks so far.
posted by one_bean at 4:02 PM on October 20, 2006

My guess is the same as above: there's something that resonates at just the same frequency as your engine idles. (The usual culprit is a heat-shield of some sort. I've had a similar problem on VWs, BMWs, my current Mitsu, and others- and the problem has been a heat shield every time. I think it's because they're cheap sheet metal and are just barely bolted on.) It could indeed be many other things though.
posted by wzcx at 4:15 PM on October 20, 2006

I bet max is right.

A loose bracket, such as that which fits around the exhaust or holds cables in place, can drive you insane if the screw falls out.

They can get hot, along with many other car parts, so look for them when the car is not fully warmed up--and don't touch anything unnecessarily down there..

From a FAQ: I've noticed an odd rattling noise, what could it be?

posted by Phred182 at 4:23 PM on October 20, 2006

Another thing could be that your engine mounts are wearing out. Your engine vibrates more when you're not moving, and you're dumping energy somewhere other than the drivetrain.
posted by defcom1 at 5:51 PM on October 20, 2006

On my car this is caused by broken mounts under the air filter. Design problem.
posted by weapons-grade pandemonium at 5:56 PM on October 20, 2006

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