The wife has a 2004 Corolla, that is rattling under the hood to all hell. I've used a stethoscope, and tried to identify a few culprits but my search continues. Where It gets interesting is the noise has mysteriously quieted itself. So either it's magically fixed itself or there are further problems. Any insight or experience with this magical and mysterious noise would be appreciated.
Noise changes with engine speed? Look at all the accessories. Any of the pulleys wobbling? With the motor off, of course, grab all the pulleys and try to wiggle them. Any play is a bad thing. Does that one have one serpentine belt or multiple drive belts? If multiple, disconnect one and see if the noise stops. If a serpentine, you can take it off and see if it stops, which will narrow it down to an accessory and not the drive pulley, but that's it.
In reply to Dr. Hess:
Regular serpentine belt, I will dive into it a little deeper. It does change with engine speed as well.
I friend at work had a similar issue. It was like a tapping or rattling noise and was traced to a idler or tensioner being loose on the serpentine belt. There was a TSB.. Let me check.
Found it.. Toyota Technical Service Bulletin EG021-07
http://www.z-car.com/lotus/files/2008-05-04_170036_T-EG021-07.pdf
and..
a quick google search shows about 2.78 Billion threads on the issue on various toyota forums.
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