A couple of months ago I got a custom made aftermarket exhaust for my 335xi. It's a BEAUTIFUL 3" tig welded stainless steel quad tip exhaust made by a company in Elkridge, MD. called Benchmade Proper. Pix here: Exhaust shop review and advice requested - side branch resonators
The exhaust is awesome and I love it at WOT but the drone was considerable at 1.5-3K rpm. I'm on stock downpipes (for now) and know that when I do put aftermarket jobs on it would be unbearable. I contacted BMP and set up a time for Joe and Brian to weld in side branch resonators. There are bunch of articles out there about this technique. Essentially you are creating destructive interference in the exhaust to combat the resonance. All pipes will resonate at different frequencies. The trick is to create the resonance at an RPM that you're not in often. To create the destructive interference you need to add a quarter wavelength of pipe to the exhaust. Big question. What's the wavelength. That is calculated using the number of pistons, rpm, speed of sound in air and temperature of the exhaust flow. Whatever. Fit some pipe up and if it doesn't work adjust. Longer gets lower rpm, shorter gets the higher rpm stuff.
Benchmade welded up a couple of capped resonators to my existing setup and basically eliminated the drone. It's freaking badass to have a stock at low rpm sounding 3" exhaust. Enjoy the pix and videos. Still need to get a damn gopro.