Hmm.... that's weird. It could be some electrical interference from a poorly shielded wire, or a high current wire, that closes to your front-center speaker wires. That's the only thing can think of that would cause the sound to start when the headlights are switched on or the revs are increased.
'07 335xi, Alpine White
| Cold, "Sport", and Luxury packages | Tinted | OEM Black Lines | JB4 | DCI | ER FMIC | Forge DV's | Custom exhaust with secondary cat delete, Magnaflow mufflers, black Dimisa Tips | Staggered 18" V718's, Matte Black | Flappy Paddles | Titanium CF Trim Wrap | BMW Performance Shifter and E-Brake Handle |