short story:
for a short time today my car made a very loud slipping-belt-style squealing sound from what sounded to be the LH side of the car, most loudly under deceleration. Engine was 90% warm when I noticed the problem, and continued on for at least 3 or so miles, and then stopped when I returned to my workplace.

LONG STORY:
on AIM, with Maximahappy22
Me: i was sitting in my car [after i got off work], with it off, messing around with some interior stuff [at the parking lot of the place where i work]. then i finished that up and readjusted myself for driving position instead of leaning-over-and-messing-with-stuff position, turned it on, waited a minute for warmup/oil, backed it up, pulled out and around and started on home. i got caught behing a cat bus [CAT = Capital Area Transit... the public transit system in my city] so i took a sidestreet and as i was going slowly, road noise was minimal, so i could hear everything else way more clearly. and what I heard was a squealing sound... now there was another old car in front of me so I figured "oh it's just that volvo" but then the volvo turned and i was alone, and the noise was still there... so it was my car. it sounded like the sound of slipping belts, except it was on the LH side - you know, where the TRANNY is. was loudish under acceleration, but REALLY loud when coasting
maximahappy22: its a loose wheel
Me: i was totally embarrassed, but moreover, worried about my car
that [a loose wheel] would sound like a squealing belt?
maximahappytv22: well idk. not really. sry continue
me: so i put my window down and was listening to it, and passed a BMW and it echoed off his car super loud and so I decided to head back to work and look around where i had been parked. so i squealed my way down a few more streets, and SQUEALED my way down a long incline, and turned into the lot at work. as soon as I did, the sound stopped... i think. i know it was squealing madly down that hill just a block away from the store, but i don't know if it squealed its way through the parking lot or not. so i went to the window and said "hey listen to this when i pull around again" and so i went around and sped up and turned (understeer, naturally) and was still going 15-20 after i finished *not* turning (ha ha), and then turning, but no more squealing. so was like wtf and went to my grandpa's house nearby and no squealing over there either. so we checked my tranny fluid and i looked and there was nothing appearing messed up that i could see and i tested it again with them standing there (up and down the circle) and nothing, and when I left tonight, still nothing just my stupid hesitation [as noted and discussed here: http://forums.maxima.org/showthread.php?t=509650 ], 30sec after startup, but that's an unrelated problem. fluid is properly filled and mediumpinkish in the A/T, maybe 6000-10k miles on the tranny fluid, looks like. Of course, I've only owned it for 4k miles.
END OF LONG STORY

OK so. Does anyone have any guesses as to what this may be? My grandpa figures I just got a branch or something somehow lodged under my car, and that I jarred it loose when I did that bit of understeer-sliding through the parking lot. but I honestly thought the sound was gone even before I did that.

the only other tranny-related problem i've ever had is that sometimes it will wait WAY TOO LONG to initially enter OD after warmup. I mean engine fully warm and cruising at 45mph, and it still is plugging along 2300rpm in 3rd, until it decides to go into OD. And just one time, I slammed it into park when lying down across both front seats doing something and thus unable to stop the car by any other method (5mph tops, engine off) and just once I shifted into Drive with the car rolling backward (2mph tops), because i meant to go into Neutral but my hand slipped and accidentally pushed it too far and into Drive. But otherwise the transmission works fine. I noticed no major noticeable performance issues today, just noise. Under some acceleration the car ran as high as 2100rpm in OD @45mph i guess because the lockup wasn't engaged and i was going slightly uphill or something, but normally 1800-1900rpm in OD for 45mph.

UPDATE: no noise today either, but the RPM seemed about 100 higher than normal. i'll put my accurate speedometer back in tomorrow to make sure. OD worked fine today.