The cold hit hard here yesterday. Last night I was stopped at a red light and I could hear a squeal coming from the engine bay. The lights were on and the heater fan was on high. I got home and checked under the hood and realized that the sound was coming from the alternator. It only happens when the truck is idling and the alt is under a heavy load(lights and fan on). Is this a sign of a weak alternator? Or maybe a dying battery? I can replace both for under $200 so I may just do that(both the battery and alt are over 6 years old) and eliminate the need to troubleshoot in -30' weather, but I'm just curious what might be causing the alternator to squeal.