Greetings! I am new in this forum.
I have a '99 SR5 4x4 (auto trans). I have had this vehicle for over 2 years with out any major problems. Lately, I noticed hearing a knocking or continuous "thud" sound coming from the middle part of the engine (or it might just be echoing from anywhere in the engine compartment) . This sound only occurs when the AC compressor kicks on (cooling or heating) while the transmission is engaged in Drive or Reverse (NONE on Park or Neutral). The "sound" continuous while driving as long as the compressor is engaged.
I initially thought it might be some bad bearings in the belt pulley or in the AC compressor clutch. But I was wondering why the "sound" does not occur or goes away upon shifting to Park or Neutral. If any of you might have experienced this or might have an idea what broken part or something that is causing this continuos "thud" sound, I would really appreciate your help/advice. Thanks in advance for your help!!

 

hmm..interesting...since it only does it in D or R, with the AC compressor working..it sounds like its something that is caused under load...have you tried having someone else put it in gear and hold it there while you listen?

 

I did try to listen, and I could hear the sound coming off the engine somewhere. However, I have not been able to exactly pinpoint the particular part the problem "sound" was coming from or causing it. That's why I thought I'd ask if anyone might have also encountered the same thing. The only thing I've been able to really confirm it occurs only when the vehicle is in gear. I tried to listen close to the AC compressor location but could not feel it there. I am continuing to observe closely. Hopefully, I'll find the problem soon, be able to fix it and will post it here also.
Thanks again in advance for any further help or advice.

 

Iam having the same problem, but my 4RUNNER is two wheeled drive. There are no sounds while it is in motion just when I come at a full stop in "D" or "R".