noise in pulley area

[79 300CD]

I have a noise coming from pulley that is adjacent to compressor. I don't exactly know if this is the idler pulley or tensioner. I know that when the compressor is engaged this pulley stops moving and keeps a noise. The pulley on the compressor seems ok. My question is: Is the compressor causing this problem or could it just be the adjacent pulley assembly. When I cut the a/c off the pulley never stops turning and the noise goes away. Please help this novice out!!