I just wish it was easier to post pictures......I could sketch what I mean.....I always make my own pipes and experienced this in the one setup I had.....it's definitely what I said ...it's like a flatulant type of sound on deceleration. On deceleration the air is drawn back into the pipe and hits that flat brick wall and rattles everything..
https://www.google.com/search?q=insi...Nw7dT2Ka4BxpxM