Ok after investigation it turns put my backbox has a blockage.
It has the standered gti down swept backbox. Got my mate to rev wile I put my hand over them. Only 1 of them has pressure coming out of it. The other has a little farts worth coming out. So no sure of the piping inside them back boxes but one side is blocked. Tried sticking a a really long screwdriver down it but still wasn't long enough to reach anything.