So looking around Amazon I came across this electronically controlled butterfly valve which basically bypasses the rest of your exhaust system as you would want to install it towards the headers area.
http://www.amazon.com/Godspeed-Perfo.../dp/B00892WGOO
Besides creating an obnoxious ruckus won't the loss in exhaust back pressure actually decrease performance at the benefit of sound?
And with a 335i F30 I would need two.....might as well get a DP does the rumble I want and the power I need!