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.
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'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 |
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