Does it happen with foot on the clutch whilst moving. ie, free-wheeling, or is it harder/softer when braking?
Have you checked the obvious like a stone stuck somewhere. I once had a stone stuck in a vent in a brake disc and it was knocking against it's back plate.... But then I'd assume it would be felt in the steering??
Worth a check tho? and run your hand over the tyres to see if there's something stuck in the tread?