2001 Lexus RX 300 REAR SHAKING

My car is fine, engine-wise, but ive noticed that while im driving, the rear part shakes, it sounds like its coming from underneath now that im listening to it. i thought at first it was coming from the extra wheel kept in the back but i once removed everything from the back and tightened the bolt holding the wheel down then drove around but it still made that same rumbling sound. so i concluded it must be coming from underneath the car.

the funny thing about these shakes is it only comes on when i go over small bumps and grooves in the road but if i go over a major bump or pothole, it just makes the normal bump sound or a lil shake or rumble, much smaller than if i went over a small bump.

also, my gas pedal squeaks, my steering squeaks a lil when i turn it a lil too quickly, my car doors rattle when the speakers are too loud but im guessing that is normal....