i would suggest that next time you hear the noise you pop upstairs sharpish and ask what they have done over the last few seconds up to the last 10 mins or what may be operating like central heating 'washing machine 'cooker 'dishwasher
you can of course narrow this down to the area below where you get the noise but don't rule out other equipment or actions as they may be connected[via pipework for example or just manifesting its self in the one area although remote]
ooo and dont rule out noisy doors
and deffinatly dont assume its dripping just because it sounds like it