Under the hood noise... (?)

I have a 95 3.4L and there is some noise under my hood. Sounds like a chain being dragged across gravel or a bolt on a plate of metal that is being vibrated. Dad says it sounds like the A/C compressor. Only makes the noise when in gear. Not coming from the trannsmission area. Its on the lower left back area of the engine compartment if you're standing in front of the car centered.

Any ideas what it could be?

Can anyone tell me the order of the pulley system starting at the harmonic balencer and going clockwise? I think I found the loose part

There should be a diagram stuck somewhere under the hood.

Otherwise, going clockwise from the crank pulley:
1) Under crank pulley (ribbed side)
2) Top of belt tensioner ((smooth side)
3) Under AC compressor (ribbed side)
4) Over altenator pulley (ribbed side)
5) Under water pump pulley (smooth side)
6) Over power steering pulley (ribbed side)

But why does it make noise only when in gear?? Those pulleys are always turning.

Could be a loose bracket or missing bolts and while the car is in gear it is under a load and makes the engine vibrate a little more.

Well. I dont think its the pulleys after all. Took it to the local autozone and they scanned it, altought it didnt come back with anything. The tech guy there says to change my transmission fluid and filter. Seems to be louder with a bigger load on the engine like ac going, lights, but when put into neutral...it sounds fine, even with all the extras pulling on the engine.

So only in gear it makes the noise. Add more of a load...that sound picks up and gets louder. Dad still thinks its the A/C compressor. I'm starting to think its the trans. I can try to get a video of whats goin on and post it on you tube for your veiwing pleasure and to see if you might know what it is.

oh and as for the diagram...There isnt any stickers or labels under the hood. Someone took them off or maybe the heat and elements made them wear off.

http://www.transmissioncenter.net/TransmissionNoise.htm

The torque converter part sounds right on the money of what problem Im having. Will a transmission fluid flush and refill and new filter fix this issue?

Ok, nvm. I found the problem and it was the catalytic converter. Apparently I hit a speed bump too hard and it broke the pottery in there. Gonna get it fixed soon. Thanks all for the suggestions and help.