Hi- I have a fully deleted diesel. It sounds great, and I still have the factory muffler. It sounds more throaty from outside, and you definitely will hear the turbo whistle with the windows open. With the windows closed, you'll barely hear any difference. I kept the factory muffler beacuse I wanted it to be about the same sound level, though I'm thinking about putting an electric cutout in to be able to have an open dump to be louder.

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'10 X5 35d- 279k mi, deleted and DUDMD stage 2, daily and tow vehicle
'01 330i- Drift car: SLR Super angle, BC coils- keep it simple
'07 335i 6mt - Sold @ 171k miles- stage 2 daily commuter for 2 years