Are you sure it isn't something dripping. We had an annoying persistant sound for weeks. Hunted high and low for source. Turned out to be a leak in the neighbours upstairs radiator pipe, dripping against the connecting wall!
How loud is it?
Wasps make a kind of ticking(ish) chomping noise when they are building a nest...
i have no idea if wasps are alive this time of year though.... we had a noise in our sitting room and i told my mum we had wasps and she didn't believe me, but there was a huge nest in our attic!
we also used to get a ticking noise from where the pipe to the radiator went up through the polystyrene ceiling tiles and made them move!