so my dad was driving the car and we were at a light. being the smart one he is he decides to slip it into nuetral, and rev the engine to 4000, then drop it back into drive and pin the gass to the floor

. after that, the flexpipe was fubared. so we got that fixed, and now the car makes a wierd griding/rattling in the front end, only upon acceleration. what is it? n e one know?? i want to get this fixed asap, as it's really annoying. any help would be awesome! how much would this cost approx.?