87, 2.5l, 5-speed. I hear a squealing like my belt is worn, but only sometimes: When i first start the engine on a very cold day is the most frequent, but it also does it when if i'm running the heater full blast and come to an idle, it will start to squeal. If i turn the heater down to medium-high or medium-low, the squealing will immediately stop. Could it be a worn tensioner similar to an issue another user was having in another topic?

belt or tensioner... blower on high draws more current than blower on low, more amperage the alt puts out the harder it is to turn, so your belt slips more with blower motor on high.

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87GTseries 1 3800sc (7.597 @88.53 1.579 60ft)
(series II swap in progress)
85GT Northstar/ 4t80e
86GT 3800 n/a
Northstar Rebuild

[This message has been edited by jb1 (edited 01-31-2014).]

A Mechanics-Stethoscope might help locate the problem or a pice of rubber hose.

If you've created the circumstance while idling with the decklid up you'll at least get an idea while standing back there listening to the engine too.