The noise goes away with AC off. When accelerating the noise also goes away. Makes me think its the drive belt. Ideas how to check this at home?

 

sounds like a bearing is gone in the AC unit or as we say use it or lose it . george

 

Today I took a closer look under the hood. The drive belt for the compressor feels a bit sloppy. I can deflect the belt about 1/2 inch by pressing with just two fingers. I would expect nearly no deflection at all, but I might be wrong. There is not much room to use a belt tension gage. I wonder how involved it is to tighten up the belt. Or to change it altogether.

 

Not too difficult - takes nothing more than replacing the belt tensioner.
That is unless George said, and an idler pulley bearing has failed and is causing the slack.
...you can't manually tighten it. The tensioner keeps it as it should be.
And yes, it would be a good idea to replace the belt at the same time if it's getting old.