Rattling noise Bose subwoofer

I have recently demounted the rear bumper (B7 avant) and found the cause for the rattling noise outside the car when playing music with a lot of bass.

There are rubber flaps in the ventilation channel on each side of the car and the one on the right hand side gives a rattling noise when playing loud music.

I checked with the Audi garage and they said that the flaps have a mission and should not be removed.

Anyone who knows how to fix the problem?

I replaced the rubberflaps with new ones.
I went to a special store that sells different qualities of rubber and purchased a slightly thicker and softer rubberquality.
I then cut new flaps with a pair of scissors and made the holes in the right places.

The bumper is now back in place and the rattling noise has almost completely disappeared.

I can now wash the car and enjoy good music also from the outside of the car