2004 Citroen C3 noise at 17mph!?

Hi can anyone help, my recently purchased 04 manual C3desire has started making a noise but strangely not all the time! At 17mph and then in the 20-30mphs it makes a short 2-3 second grinding/sc noise. It doesn't sound like it's coming from the front but understandably I'm a bit hesitant to lean too far out the car when in motion just to hear! It doesn't do this every journey so taking it to a garage is a bit of a random chance of it doing it. Does anyone have any ideas? I'd be really grateful - I am just reaching the end of my 3month warranty on the car so would like to get it fixed under this but the garage is 30mile away and sods law says it wont do the noise if I take it! All ideas welcome!