2004 '3, 160km, great little car (except for the typical Mazda rust problem). Okay, here's an odd one.When the compressor kicks in, there's a buzzing sound -- as if there's something caught in a cog somewhere, or a belt's teeth riding against plastic. The buzz rises and falls in pitch with tach revs...but here's the odd part...it only does this in Drive. Not in Park, not in Neutral.

I've got under there with my flashlight, but can't see anything unusual. Anyone heard of this before? I hope I don't have a major bearing problem...

Thanks!

 

Thanks elmerkit, much obliged. Tension seems okay, so I guess I'll have a bit of wrench work to check the idler bearing. Hope that's all it is...the way my luck runs, if there's 10 possible things to go wrong, mine will be the most expensive to fix.