Ham fisted chaps , had mine for 5000 miles , not a problem. Yes the chin vent on the Sys.5 is noisy ,in 3/4 years of ownership never used it, dealer should sort you out with a new part.
Answer is no BMW do not sell the vents separately, the forehead vent broke on my System 5 when it was 23 months old and as a result BMW replaced the lid under the 2 year waranty