Well that's obviously completely up to you. You should be in and out of the dealer in 30 minutes for this. Mine was doing the same thing, but I have mods I'd need to maybe take off before a dealer visit, so just held the flap open with some wire and the rattle is gone and the flap is open 100% which I actually prefer.
If your car is stock, I'd probably just have them fix it, why not?
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'15 M235xi | BSM/Oyster/Aluminum | Black Kidneys | RW Carbon Mirror Caps | 40% tint
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