Rear suspension noise

[CygnusX1]

I've had my 2005 Camry for about 10 months. It's always made a funny groaning sound from the rear when I go over a speed bump. The colder it's gotten here in Phoenix, the worse the sound is. This morning it was in the 30's (hard to believe) and the sound was loud even when I went over small bumps. The sound was even there every time I hit the brake or gas pedal. I have nothing in the trunk BTW, even though it sounds like something is bouncing around back there.
The dealer heard it this morning and told me it's the rear struts/shocks echoing though the unibody. Nothing they can do. Whatever. I've never heard that kind of sound in a car before.
I read somewhere here that regreasing the bushings could help.
Anything else I might be able to do to get rid of that sound. Tempe Toyota blows so I might try another dealer.

Thanks.

[Gen5.5Cam3MZ-FE]

this is what happen to me i remove my rear stock sway bar and install the trd sway the next day in the morning it started to make noise only when i go over dump or pot hole so i reomove my trd sway and pack with more grease on the bushing and now there no more noise in the rear so i think you just have to do is add more grease on your sway bar bushing