I Had the same problem and went bact to the dealer a couple of times to complain.He first said said it was because it was new and the noise will go away.It turns out there is something wrong with the rear bearing and Strida is aware of this and has recalled many of the recent bikes because of this problem.I was told that they will change it [The bearing]and will take it back over the weekend.I have only had the bike a couple of weeks[Strida 5.1] and it is a pity that that this occurred to an otherwise excellent product.