The sound is probably from the plastic covering hitting the rear deck. You will have to put something in between the area that is making the noise.

TUrn your music up so that it makes noise. THen get into your backseat and listen for the area that is making the noise. You can press onto the area you think is making the noise, if it stops...then you can stick something soft in between the covering and the rear deck. This happened with my car, and I end up putting a piece of this fabric in between...something like a sock.