the timing belt and tensioner were changed 5 months ago. and besides, why would the sound only come on when the compressor is engaged?

 

Because the water pump fins break off making exactly that sound, turning on the A/C just increases the load on the engine. I've seen techs misdiagnose a bad water pump for rod knock, valve tick, cam tensioner, bad pulley etc. the list goes on and on.
I would just keep that idea on the back burner while you're doing your diagnosis of all the other pulleys, if you can't find anything wrong with the serpintine belt system or valvetrain, then thats probably what it is. If the car starts magically overheating as time goes on then its not a guess anymore. Watch your temperature gauge, if you see it start creeping past the 1/2 mark........

I would still start with moving the front end forward and doing the old screwdriver stethescope trick.

 

Ok, Thanks auditech. I was just at a friends house and its cooled down outside and when i started the car (without AC on) it made the sound, this is the first time it happened w/o AC on. it got louder when I turned the wheel from lock to lock. I think im going to try pulling the accessory belt off tomorrow and start the engine cold to see if the sound disappears to discount the waterpump and drive it to put some load on the engine. do you think this would be a good starting point?

Thanks again for your help!

 

Sure, thats exactly would i would do now that you verified it without the A/C on.