Q: My car is making a squealing sound when it cranks and when I turn on the A/C

Hi, thanks for writing in. What you’re describing sounds a lot like an AC belt that is loose or the serpentine belt on your alternator loose (or a combination of both). A common mistake that some mechanics make is not properly tightening the alternator after replacing the belts. The fact that the previous mechanic indicated that there may be a bolt missing also raises a red flag to me that the alternator might be a problem.

However, you mentioned also that the car squeals when you turn on the AC and that it gets louder as you drive. When a belt is loose it creates a harmonic squeal, and as the belt moves faster (when you accelerate) it progressively gets worse.

I think the best thing you can do is to find a another mechanic that can fully inspect your alternator and AC belts, replace any lost nuts or bolts on your current alternator, and get rid of that annoying squealing sound. A certified technician from YourMechanic can come to your home or office to inspect you AC squeal and let you know exactly what needs to be done.