Did it start happening right after you changed the tires? It's highly discouraged to have different tires on the same axle. Especially if they're not the same diameter or belt pattern. If it just started out of no-where you might need your tires balanced, maybe an alignment is in order OR it could also be that your driveshaft center bearing's rubber has dried out and fallen apart.
*disclaimer*: all that I say may be true or I may be completely wrong and I accept that.... please, it is your duty as another person on the internet to tell me (in-politely) how wrong I am :) "The dreaded 2nd windshiled wiper swipe when you turn it off its almost like tis showing you that it's boss, and it doesn't have to stop when you tell it to :("-College Boy This is what happens when you leave your Z parked outside too long:
 |