I actually found this interesting information about Helmholtz Chambers/J-Pipes on M3Post (So I started a thread about custom Magnaflow mufflers... - BMW M3 Forum (E90 E92)) and plan to try this method once I get my Magnaflow 14816 muffler installed because they're notorious for droning. I just hope there's room to add Helmholtz Chambers/J-Pipes in a proper place for E60's because it looks like the middle of the car where you would add them is kind of blocked by what looks like could possibly where the spare wheel sits.