I'm getting this vibration inside the car only when the ac is on. What could be the problem? When I turn off the ac, the vibration disappears.

 

Compressor clutch.

 

any noises? take a look at the compressor when on/off how is it vibrating and does it look like the mounting is proper? Could be the compressor, the clutch or that the idler pulley has a bad bearing as well - you'd need to remove the belt to turn the wheel by hand to check that. Anything else under the hood that appears to be vibrating or resonating?

 

I'll check the idler pulley tonight. So what am I looking for once I turn it by hand?

 

see if any cooling fan blades are missing a piece (on many cars ac turns fan on, I think xc90 is one)