The noise could be coming from the seat post where it clamps up. Back in 2008 I bought a brand new MU SL and within a few months the wear on the frame hinge got so bad that you could place your hand under the frame, lift up and see the handlebars and seat post move away from each other, this was despite careful adjustment of the hinge clamp. The bike was replaced under warrantee and knowing what to look for I noticed the same happening again within a month or so... second bike was replaced with a 2009 model which was found to have a nasty chip out of the frame hinge... I was not prepared to accept a bike in that condition so sent it back a and got a refund.... I "cured" the problem by buying a B-spoke S-type Brompton.... four years on and the Brompton has been fantastic, no squeaks, rattles, frame wear.... best thing I ever did was to buy the Brompton... shouldn't have wasted time with Dahon, should have bought the Brompton in the first place.