the past couple of days i have noticed a strange groaning sound coming from my engine compartment. it seems to make the sound right before the engine warms up completely. also the groan turns into a high pitched whistle/squeal when i give it gas. when the engine is completely warmed up, the sound it gone. any ideas on what i should look for? im thinking a belt but wouldnt it make the sound continuously?

V6 pickup 4x4, 385,000 miles and still ticking