I had my ac belt and my serpentine belt chaged on my 2007 M35 now my car is making a horrible noise.When I start my car then after a few minutes it stop.But I notice if my ac is on it makes this noise also what could that be?
When belts are installed, they need to be adjusted (depending on the vehicle) or they can slip causing a squealing sound.
In some cases, the new belt is not stretched like the old one. This puts added pressure on the idler pulley. That can create a lot of noise if the pulley is going bad. They should have checked it.
From what you describe, it sounds like they improperly adjusted the belts and they are slipping. Take it back to the shop that did the work.