Help with diagnosis

[G-83972]

My GMC Sierra 1500 started to make an intermittent metal squealing noise when driving. It seems to happen anytime the vehicle is in motion, every few miles or so, but not as a result of any particular movement. It has gotten progressively worse, and now sounds like an elephant's trunk, with a jump in RPM for about 3 seconds. My mechanic checked the wheel bearings and brakes with negative results. Any ideas?

 

[98trck]

A/C clutch ?
rpm increase, it is not unusual for the rpm increase when compressor engages
on again off again, compressor engage - disengagement
a/c compresor, also engages when set to defrost, also if set to recirculate air


PETE