creaking with crank arm

i've done about 1500km with an Orbea with campy kit on it.
when i was sprinting up a hill the other day a very noticable creaking sound came from the bottom bracket area on the left side.
then after a couple of pedal strokes it was back to being silent again. there is no noise at low revs in any position but as soon as the revs increase the creaking comes for a couple of seconds.

Any ideas what it could be?

thanks