Depends on how intermittent, and whether it's rhythmic or random. If it's fairly steady happening every 2-3 pedal revolutions, I'd suspect a stiff or damaged link in the chain.

Otherwise if it's more like every pedal revolution it could be bottom bracket play, but if you didn't have it before and didn't touch the BB, hold this as a last resort.

More random noises, might simply be that you didn't tighten the cassette lockring enough, or that the cassette didn't overhang the freehub body, so the lock ring isn't compressing it properly. Stick a spoon between 2 sprockets, and see if you can lever them apart slightly. If so the cassette isn't tight.

That's just the simple obvious stuff for starters, check that, and come back with a better description of the sound, and it's frequency of occurance, ie random, once per pedal revolution, only when under load, etc. and we'll give you more things to check.