I have a new S75D with under 300 miles. Tonight I noticed a high-pitched sound (a squeal or whistle) that vibrates at around 49 mph, becomes steady at, say, 50-51 mph, vibrates again at around 52 mph and then goes away by around 53 mph. It feels like is coming from behind. I also believe there is another incidence at a slower speed. This seems unlikely to be wind related as (1) it is not an air-rushing sound, (2) it goes away as speed is increased, and (3) the noise vibrates before becoming steady. Does any one have an idea what this might be? Thanks.