if the car is lowered enough for it to bottom out the suspension, like mine does, you wont be able to get your hand in the wheel arch, or under the car sills, and will barely get over a speed bump without scrapping the bumper. lol.
So we can count that out.
Dont think its the suspension bottoming.
Is it metal onto the road scrapping? Have you checked to see if any of the exhaust is hanging low? Broken? Noises?