Would like to know what causes my truck to squeal when it first start it up on 1992 Nissan Pickup (D21)

The odd thing about the whole thing is that when it makes that noise I usually turn my lights on and off then it stops or when I turn the heat off it stops. But it does it when it has sitting for a few hrs. It last about 15-20 seconds then it stops.

Tighten or replace the belt that drives the alternator and have battery checked! Extra load on the alt. after starting the engine due to drain from battery, after alt. charges the batt. a little, load decreases and belt squeal stops!
1 more answer
rong size belt.or just needs to be tightend