my cooper s runs really smooth upto 3400rpm then feels a little hesitant and a little rough, it still accelerates really well but just dont feel quite right, its not a misfire or yoyo as ive read on here, ive had a play with the valve on the supercharger and that seems fine, it closes nicely with a little metal to metal ting, i have had it on a mates diagnostics but everything fine, is this just a characteristic of the car?, any ideas?, i did speak to lohen about fitting new coil/leads/plugs but they said they wouldnt really cause that sort of problem

Roll down the drivers window and floor it in 1st, 2nd and 3rd, can you hear a rattle as you go? If so it's being caused by the harmonic damper on the front of the engine, behind the drivers wheel. Characteristic of this is that you can't hear it with the window up, but can hear it with the window down!

I've got this issue currently - about to swap out the harmonic damper this weekend...