Hope someone has an answer to this mystery.
My friend bought a new 2011 WRX sedan and from day 1 she can hear this knocking noise coming from the shelf between the rear seat and rear window. She brought it back to the dealer and they tried to fix it but the noise is still there. Coincidentally she also has another friend who bought the sme model and is having the same noise problem.

Does anyone else has the same problem and if so, how was it fixed?

A mechanic at the dealer try to explain to her it's because of the difference between the noise frequency of the exhaust and the frequency in the car is causing the shelf to vibrate etc etc etc.....don't know if it's true or not.