With any luck it's just a loose fixture point on the dash. Have somebody else drive over some ditch witch situations and use your hand to feel for the source of the sound.

Sound travels much faster in plastic (dash) and even faster in metal (frame) so the source can be very hard to find when you are confused by the sound arriving much slower though the air.

If you track it to inisde the cabin/dash, try using shims to bind the dash so it won't move enough to pop. If you can do that you will determine it's a loose fixture. Might mean you have to remove the dash to actually fix it but that's much better than some options.

Also: even if it's worst case situation like the spot weld horror, there are ways to resolve if you can find that as well .

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)