A/C problem

I have a 95 Sonoma 4x4 with a 4.3L Vortec CPI. When Idle and the A/C is on something under the hood will rattle very loudly. Once you get up to speed its gone and when the A/C is off it dosnt rattle. What could be the problem? Im thinking it something wrong with the A/C belt.

the clutch on the a/c pump needs to be replaced

could be the pump itself. mine done the same thing. it got so bad, the pump was bogging the truck down. eventually the pump slung its rods out of the housing. if you have a leak and you have to add freon often. make sure you keep oil in the system. (never put oil in the low pressure side, turn the truck off and put it in the high side)