does this sound like a bad (or going bad) alternator?

my jeep has been whistling at me from under the hood for a while now, but it seems to be intermittent and not have any rhyme or reason as to when it whistles. i haven't had the chance to check it out in detail, mainly because i don't drive it that often (i really only drive it when it snows, as its my winter vehicle).

today we got some snow, so i'm driving the jeep...i hear the whistling like i always do - but, i notice my turn signals are extemely slow. so, i decide to start doing some testing...i turn my blower, rear defogger and wipers (both front and back) off, and test my turn signals again. they're a little faster, but not normal speed yet. i start driving (this was while sitting at a traffic light) and get up to normal speed (about 35-40 mph) and test the turn signals...they're working much closer to normal now.

i get to where i'm going (by this point the whistle has stopped, it stops most of the time once the engine gets warm), and when i come back out there's no whistle (i wasn't really in there long enough for the engine to completely cool down). just for kicks, i turn the blower on...no whistle. i turn the wipers on...no whistle. i turn the rear defogger on...almost instantly, the thing starts whistling at me again.

the only other thing i've noticed is that it does seem to happen much more often when it rains than when it doesn't...thoughts?

Is the belt loose? The 4.0 likes an extremely tight belt.

good question. my friend is doing some work on the jeep for me tomorrow (it's too damn cold for me to even think about doing it myself), i'm going to autozone to pick up the parts...i'll have them check out the alternator and then have him check on the belt tension when he does the rest of the work.

don't overtighten the belt, it may cause the alternator to go bad. I believe thats what happened on mine.

Whistle might be a vacuum leak. It could be temp-related if it is a small leak near a flexible rubber segment. When the rubber is cold, it is rigid. Once it warms up enough to make the rubber flexible, the vacuum might suck it down and close off the leak.

As for the rear defogger, if that is a consistent pattern, try the front windshield defroster setting as well. I know that the defrost setting causes the a/c compressor to cycle on, and presumably so does the rear defogger. If both settings cause the whistle but only when cold, that might be a hint as to where to look for the vacuum leak.