I own a 1998 Toyota Corolla LE with ~142,000 miles (manual trans). I just got new tires, wheels, brakes (and rotors), and had an alignment. Assuming the tires are balanced correctly, why would I experience a serious vibration at 70mph and faster in the steering wheel and floor? While driving with the cruise on at about 67mph, the vibration's intensity fluctuates over a few miles-- sometimes the vibration is bad (visible steering wheel shaking), other times, it is barely noticable (not visible, but can be felt). The vibration seems to be noticable mostly after a slight right or left corner on the highway. I can't drive at 70mph because the vibration is too intense. Also, perhaps this is related to my problem: when I go slowly over bumps I can hear a slight rattle in the front.
What could cause the vibration? Is it going to be an expensive or serious fix?
Thanks.