it dosent do it when i have the heat off but it does when heat on? any suggestions.

 

belt sounds loose or ac clutch bearing going bad

 

see if your belt is cracked or glazed. If so, replace it. While it's off, check the tensioner pulley for play and noise and replace if necessary. If the noise only occurs with the control on defrost, then it's the clutch on you a/c pump which engages automatically when put on defrost

 

only does it on defrost

 

your ac compressor runs when defrost is on thus putting more stress on belt