I get this rather bothersome noise from the rear of my '08 B7 when the gas tank is below 3/4 to 1/2 tank. It's the gasoline rocking back and forth hitting the tank walls and creating 'clunking' sounds especially when I'm in a stop and go situation (traffic jam). My initial thought was the rear brakes, but I'd realized this after when I filled the tank to 'Full" and it stopped - then comes back when the level of fuel reduces causing the liquid to bounce, thus creating this 'clunk...blub...plunk' kinda sound.
Anyone experience this?

 

I have the same thing. I think its just the gas rockin back and forth.

 

Have it checked out. Usually there are baffles in there which prevent sloshing. If one of the baffles is loose it can cause the gas to slosh and cause a clunk when it hits the front of the tanl.