So ive been having this sound for a while now, and just seems to be getting worse. ive got a full exhaust minus the headers from Fast intentions and i love it, only problem is sometimes at idle, normally when the ac is at full blast or im sitting at a light in drive (slightly more load on the engine than in neutral) i get this noise. it goes away when i put it in neutral.

I got up under the car and could basically make the noise go away by pressing on the rearest most bushing, the one that holds the actual muffler up at the rear of the car. im thinking its this causing the noise. Ive searched everywhere and cant seem to find the replacement bushing for this part. anyone ever replaced these before?

 

check the bracket that goes across the pipes. a buddy of mine had the same issue kinda, he added a few washers so the brace wont be so close to the pipes.