The flap is on the nearside exhaust pipe tip.
It is most likely to be the actuator and hence a cheap fix. Get under the car and apply vacuum to the actuator and check the flap closes/opens...
If wanting to replace the backbox alone I'd go with a BMW performance system.
Do you have catless downpipes? Bastuck back box sounds awesome with CL DPs!
I wouldnt replace mid pipes unless you want it LOUD...
2008 E90 335i LCI M Sport / Manual / Black / Red Leather / Avon ZZ5 / VMR710
Mods: JB4 G5 ISO / BMS Back End Flash / BMS DCI / AR Design DPs / ETS FMIC / Meth Injection / Spec Stage 2+ Clutch / BMS OCC / Cool Carbon Brake Pads / RB PCV Valve / Forge DVs / M3 Control Arms / Bilstein B4s / Eibach Pro Springs / Eisenmann Race
Wish List: LSD