Clicking/ticking noise under acceleration.

My car has developed a ticking noise in the last few weeks. It only happens under acceleration. It seems to happen accelerating from a standstill up to about 50 km/h and then it just stops. It doesn't happen all the time. Sometimes it will do it, sometimes it won't. It tends not to do it under hard acceleration, although it's possible that I'm just not hearing it over the exhaust/engine noise (muffler delete). It occurs most regularly under normal acceleration away from the lights.

I just have no idea where to look as I can't imagine anything that would make that noise.
Coincidentally, it began shortly after I had the water pump replaced. I doubt it is related, but if that helps anyone come up with an explanation, it's worth mentioning.

It only happens under acceleration and is not related to engine speed. There is no noise when the car is revved whilst stationary. The noise is difficult to describe, but it is a constant clicking/ticking sound (not so much a knock). It seems to increase slightly as speed increases. Once the car reaches about 50 km/h the noise just stops. There are no vibrations or any other symptoms.

Any ideas where to start looking?

Clicking/ticking noise under acceleration.