My S2 seems to hit flat spots when the 'pedal is to the metal'

When accelerating (not sure what rev range it is ... keeping eyes on the road is the main idea) there's a point when the car just doesn't gain any speed or revs, and the exhaust (pretty sure its the exhaust at the rear) makes a deep burring sound ... then after about a second (cant be much longer) it stops making the sound then the car pulls even quicker then it was when I put my foot down.

Anything to look out for, or listen to ? Or anyone with ideas of what it could be ?

If it's the clutch ... then I can understand as it needs changing if I want to put my foot down.