When I start up I get this hum noise and a slight vibration, only lasts a second and I don't normally get if again. Only at start ups. I changed the belt and the belt tensioner. Charged the A/C and replaced fluids and filter one weekend and since then I get the noise. Figured the compressor might be locking up cause of to much freon. Shut the A/C off on next start and it did it anyway. Belt tensioner appears to be doing its job ok. I haven't cheched the fan/hub assembly yet and I haven't checked the A/C clutch yet. Figured I would run it past you before jumping in both feet. Thanks

 

Did you totally shut off the AC or did you turn it to the dual inlet setting? Not sure if you knew but the compressor still runs on that spot even though there is no indication for AC. I know this sounds like a stupid responce but sometimes the simple things can get over looked.

 

Could it be the Lift pump?
It's supposed to run for 30 seconds on startup.

Ed

 

Might be the turbo spooling up?

The lift pump runs for 30 seconds, and continues to run while the motor is running.

Merrick Cummings Jr

 

maybe unhook all your aux stuff (belt driven) and see if it dose it?