The non-wing 2 Series mirror caps shouldn't whistle, if mine are anything to go by. These are the UK AutoID CF caps. Personally, I think that gloss black would have looked just as good, but weren't available back in 2016 when I got these. FYI they've weathered well and are still looking good. Current CF price GBP £189.95 and they also do a wing mirror tool kit for £9.95
AutoID website at:
https://autoid.co/collections/bmw_2_series