Can you please tell me why my A/C squeals only when the transmission is engaged in drive or reverse ?
It is likely that the belt tension on the pulley is not tightened within the manufacturer’s specification.
When you shift into a drive — forward or reverse — gear, the engine slows under the load to just the speed where the incorrect tension allows the belt to slip on the pulley, thereby making the squealing noise.
Often adjusting the pulley’s tension eliminates the noise. Sometimes a new belt is also needed.