Car failed MOT for having exhaust leak at backbox/midpipe joint, basically gases were getting out. So i've taken the pipe off, put sealing paste inside box, and outside on midpipe and put it all back together. I've run it for a while to let it set, but I can still hear a sort of blowing air noise from the rear. When it's idle it sounds and feels like it's coming out of the tailpipe, obviously in pulses? Is it normal to be sound like this from the pipe? When I rev it, it gets quite a bit louder too.