This is probably a rather obscure mechanical, so any advice gratefully received from resident folding experts. I have a Brompton S6L (SRAM hub +2-speed derailleur jobbie) and noticed a squeaking noise on returning from a longish ride. Inspection revealed the rear tyre to be rubbing on the chainstay, as if the wheel had shifted in the dropouts. However (as I discovered on removing the wheel for the first time) this can't happen on Brommies, because the wheel is kept in a fixed position in the dropouts with 'tab washers'. There doesn't seem to be any evidence of bending in the frame or wheel either, so I'm at a loss as to what has caused the wheel to seemingly migrate sideways. Some hub-gear strangeness? Am I missing something dead obvious? I'm flummoxed. As I suspect my LBS may be - having recently moved up north I'm now miles from Brommie specialists. Any ideas from folding devotees?