Sudden Suspension Sounds - Creak, Squeak, Ding!

Hi everyone! Long-time follower, first-time poster! Love PriusChat!

I have a 2005 with just a touch over 95K on it. I know I'm due (and maybe a little overdue!) for plenty of scheduled maintenance prescribed in the "Passport" between 90K and 100K but I've been pretty laissez-faire about doing much other than changing the oil every 3K and telling my mechanic to "just make sure everything looks alright" since I picked up the vehicle at 70K miles. I know - there should be some audible sighs from more responsible owners, a finger-wag or two, maybe even a reply chastising me for being so frugal about the maintenance. Probably pretty well-deserved. The original owner was nothing short of OCD with the maintenance so I consider myself pretty lucky. In my defense, however, I'll tell you: there's never been a thing wrong with the car in over 20K miles. Not a peep, squeak, growl, groan, whatever. Other than getting fairly inefficient mileage (~42 city/~48 highway - I live in a very hilly place and the hills always seem to be going "up" rather than "down" for some reason...) she's been an angel.

Then came the noises.

Three weeks ago I was driving along a fairly rural road with lots of bumps and holes when I noticed a slight creaking noise from the rear driver side area. My seats were down from moving houses earlier in the week and I read on another post that occasionally unsecured seats can make that sort of sound but when I returned them to their upright and secured position, the noise continued. In fact, it's gotten a bit worse. I checked the parking brake as well - no solution there. If I go to the back of the car and push down on the rear bumper the sound happens - creak, creak, creak.

The best way I can describe the original noise is if you were rolling across an old mattress with really rusty springs. Interestingly, the noise seemed to go away when the temperature dropped from 95*+ to about 72* over the weekend. Unfortunately, it's hot as Hades again and the noise is back. It has actually morphed into something of a hollow knocking sound that is attempting to complement the already obnoxious creaking - kind of like a muted first note of the "Marimba" ring tone.. The noise occurs while I am going through residential neighborhoods, parking lots, etc. - which is totally awesome when it's the only thing people can hear from your car and everyone is looking at it because it's so super cool.

I feel as though I have read exhaustively on this subject - could it be just a loose set of fasteners in the suspension? Something that needs to be lubed? Or, *gulp*, could it be a broken sway bar or shot shocks/struts? I am scheduled to go to the Toyota dealership down here this evening but I want to be uber well-informed about what's going on with this poor baby before they take me for the proverbial ride, telling me I need new struts, shocks, springs, frame, engine, hybrid battery...whatever. Honestly, I'd just like Toyota to tell me what's wrong with it so I can take it to my own mechanic who is much more reasonable. Also, I'm hoping my own mechanic can let me under the car so he can show me where that automatic headlight leveling sensor is so I can take it off and repair it (fingers crossed) because that idiot warning light has recently come on as well.

Sorry for the exhaustively long post, I am so anxious about this situation I can barely stand it. Does it sound like something serious with the suspension or maybe just a little tightening and lubricating is called for?

Thanks for any thoughts you might have on the matter - most appreciated!