its called fritting corrision..you can spot a loose rivet on a full scale because it leaves a black streak...the revving of the engine could have been caused by the loose muffler..pressure in the fuel tank fluxing because the muffler was flexing....tighten the muffler by taking off the nut in the back, loosening the screw in the front, position the muffler where you want it and tightening everything back down..