ITRCA

I have a vibration on my back trunk at about 3500 rpm. I gutted the whole interior so i know its not that. I checked the piping and nothing is touching. I think the muffler is hitting my bumper. Is there a way to stop this. There is an 1 inch gap between the exhaust and the bumper. And also, i noticed on my friends itr, he has a apexi n1 and it is a lot closer to the bumper (lik 5 cm) and it doesnt make that noise. Can anybody help>??

I would double check to see if all of the bolts are tight and the rubber hangers are in the right place.