I went out there and popped the hood; it was coming from the engine, so I went inside, put on shoes and opened the garage door, then grabbed my 24" breaker bar to use as a stethoscope, to find out what it was that was making the noise.
There had been that whole thing with her AC, and visions of spending $400 and two bad days struggling with replacing the compressor danced in my head...until I touched the breaker bar to the alternator. Now, when one of the accessories fails, generally you need to have a listen to each one to figure out which one it is. But I could feel the rattling coming from that alternator, which was pretty much all I needed to know.
I advised her of my preliminary diagnosis and proceeded to remove the alternator. When I got it out, the play in the pulley was bleeding obvious, which firmed up my diagnosis. (I'd had some worries that it might be the water pump.) We hit O'Reilly's; the woman there put it on the test fixture and said the pulley was really wobbling. She sold us a replacement alternator and serpentine belt (I wanted to replace the serpentine belt since the belt that had been in that car was of unknown age) and we stopped at KFC for dinner. Came home, ate dinner, spent about half an hour on reassembly...and she's whisper quiet again.
I'm not kidding: that alternator was shot. Still making plenty of juice, electrically perfect, but the bearings and the pulley made such an incredible racket....
That pulley is weird: it's got a one-way clutch in it. I thought it had come loose--unscrewed itself--but immobilizing the armature and turning the pulley didn't change anything, and then turning the pulley the other way was impossible with the armature immobilized. But there was a crapton of play in the old one, and there's none in the new one.
So, new belt, new alternator, two and a half hours--that would have run $400 easy at a shop, and we got away with half that much with me doing it. And that includes the fried chicken!
* * *
I'm not terribly surprised by this, though. When I cleaned the car out on Friday, I started the engine to move it a bit and I heard something making noise up front. It wasn't much of a noise, and I didn't think much of it considering how minor it was. It got a lot worse in the past four days! And the car has over 115,000 miles on it, so you've got to expect things to need servicing.
So, I put the alternator in, and it's a lifetime warranty part, so if it does this again, it's two hours and a trip to the parts store. No problem.