boomy

I've noticed over the last few months that there is boomy noise from my Cupra's backbox. It's mostly when reducing speed from 2,000 rpm to 1,000 rpm which gets quite annoying when I have no music on[:@] The car is book in for it yearly service at the weekend, so I also asked them to look...

Does anyone find the standard exhaust on the TFSI FR boomy? If I rev the engine at a stand still the exhaust back box seems to boom? I am just wondering if this is normal or have the back box baffles gone? thanks