This is the wrong section....
Anyway there is an activated carbon canister right behind the fuel tank, on the passenger side rear of the car, covered by the rear bumper. It is part of the EVAP system. The pump you are hearing is probably the DMTL pump which pressurizes the fuel system to check for leaks....which you now have since a) you hear a leaking pump noise and b) you have a warning light...which I bet is a SES light in yellow.
Here's a diagram and part #'s of the carbon canister I'm referring to:
http://realoem.com/bmw/showparts.do?...19&hg=16&fg=10
I doubt the canister can be resealed or salvaged. Although the DMTL pump only pressurizes the system a few psi, it can detect the most minute leakage. So you would have to be really sure that the canister is resealed.
FYI, the EVAP system is not "important." I put important in quotes because for emissions reasons it is important, however for daily driving, all it does is vent evaporated gasoline into the intake system so it burns instead of leaks into the atmosphere, contributing to green house gases.
I remember another member on E90fanatics.com had an E90 M3 track car he was parting out....among the parts was an OEM carbon canister and DMTL pump. Try looking there first so you don't have to buy a new one. All you need to do is swap it out....very easy. I can guide you through it if you want.