Knocking during fast spin on Indesit Omega 1200 - what causes it?

My Indesit Omega 1200 washing machine has started to make a knocking noise when going into the fast spin cycle (at 1200 rpm, not at 600 rpm). What could cause this? It isn't worn shock absorbers, as it doesn't have any - it is suspended by two large springs, and located by thick fixing wires. It isn't affected by the size of load either. Could it be the bearings?