So, it started with me having the serpentine belt and a tensioner and idler pulley replaced. Then it started squeaking. (This was over a year ago.) I took it back, asked them what the squeak was, they sprayed it down with lubricant, and the squeak got worse. Took it back again, they said the intake manifold gasket was "sweating". I was a bit skeptical because I'd had the intake manifold gasket replaced 40k miles previously, so I took it to a dealership and had them check it out. They said that no, the intake manifold gasket looked perfectly fine, the squeak was my water pump leaking on the serpentine belt. I figured fine, if it's the water pump leaking then I can watch the coolant levels and the engine temp, which I did, and which never changed, even through last year's stupidly hot summer and a 30 mile round trip commute every weekday. October rolls around and it's still squeaking, so I take it to *another* place where they tell me that the water pump's fine, the squeak is my hubcap, and my transmission is practically shot. Not thinking clearly at the time, and because I'm absolutely sick of dealing with this, I panic and decide to get a new car. (And the squeak was also not the hubcap [obviously], as it squeaked when sitting still.) So I take the car down to my dad's buddy (who would know these things), he drives around in it, tests the transmission a bit and says that, no, there doesn't seem to be anything wrong with it. (This should've been a clue, but you can't actually check the transmission fluid in this car, just change it, which had been done within 15k.) I'm fed up and angry with car mechanics in general at this point, so I get the new(er) car with an extended warranty. Meanwhile, I get the brakes fixed on the old car for whoever buys it and the guy who fixes them says, oh, by the way, you need a new idler pulley, which hadn't been replaced the first time. So, the squeak was the idler pulley, but it isn't squeaking anymore. Like magic.
End result: the car may need a new idler pulley.