its due a a bunch of sound deadening materal about 1/4 inch thick behind the seats. You can take off your seats and remove it if you prefer. Instructions on that would be in my audio install diy if you don't know how to remove the seats.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>