2004 Mazda 6 2.3L. This is a problem i have had for about 2 years now and i am just now getting around to finding out what it is. when the cars AC Compressor kicks on there is a loud constant knocking noise coming from the engine area. first thought was the compressor or compressor clutch. while looking at the engine looks like the compressor clutch is in the front left part of engine but the noise is coming from the left rear of the engine. but again the noise is only when the compressor is on. it is a knocking/clunking noise

 

Did you get this fixed?? I've had this for a few years now and it is starting to drive me crazy!!
Thanks!

 

Yes this is the original poster of this thread. Yes turned out to be the Tensioner Pulley. Turns out it was a pretty common problem and I believe there is a TSB about it somewhere. Unfortunately I totaled my 6 last year