2001 Toyota Corolla Noise on acceleration

Hi, I have a manual(uk) 2001 Toyota Corolla. It's making a sort of clicking noise (could be a worn bearing type of noise) but only under acceleration. So if I'm driving down hill, or on the level, it's either absent or inaubible. But if I accelerate it becomes more audible. It also varies in intensity, seeming to get worse the longer a journey is, and sometimes I can't hear it at all (on short journeys).
The car itself seems to drive ok, and if stationary, or if I take it out of gear while rolling and rev the engine, there's no sound at all, so I'm fairly sure it's not the engine. If I had to guess, I'd say something to do with the fuel injection system?, But it would be a guess.
Any ideas on where I should start looking?

Thanks Ian