bad belt and/or pulley. There's also an idler/tensioner for the alt/ps also. The bearing is the same for both pulleys. BTW, what year and engine? I just changed the a/c tensioner pulley, a/c belt and alt/ps belts this past weekend on my 2.7. I also removed the bearing from my alt/ps pulley and used the bearing from my old a/c tensioner in the alt/ps pulley because nobody had it in stock.
Yeah, mine was making a squeal for about 5 seconds every morning in the winter when it got really cold outside. I thought the bearing was going in the a/c tensioner so I replaced it. Turns out the bearing was worse in the alt/ps pulley. And nobody around here stocks it, so on a Saturday afternoon, I was SOL. Closer look showed me that the bearings were the same for both idler/tensioners, so I took my vise and a big deep socket and pressed out the bad bearing and pressed out the one I thougt was bad from the a/c tensioner I replaced a week earlier, and pressed that one into the alt/ps pulley. It been on there for a week now and no problems. Sooner or later I will probably replace that alt/ps pulley, it just sucks that they don't sell just the bearing as its not that hard to change and the pulley itself is perfectly fine.
Just some info I got while having a related problem (ps/ alternator pulley) ... Napa sells a pulley that a Dodge tech told me they were using in repairs. I've checked one out and the box contains a pulley + bearing. Not sure if the bearing will work with OEM pulley or is sold separately though. Stock pulley appears to be the better one IMHO. Also, the NAPA one may be more susceptible to belt slippage when wet (should be OK with an intact splash guard)and it looks like it could possibly rub on the timing cover.
NAPA sells two or three different "grades" of the pully. I got mine down the street from Best Auto. They are Dayco brand. They cost about $20 a piece. They are getting replaced this weekend along with other stuff. We'll see if the noise goes away. It should.
Boy, when these pulleys are ready to go they sure do let you know! The noise they make can drive you crazy. I mean you'll swear that somethin' is gonna fall apart on the engine. In my case something did though - the bracket broke and I had no car for 2 days while the shop tried to find a yard bracket and pulley. After you replace it though ...