Where is the humming/pulsing noise coming from ????

For the last few months there has been a pulsing/ humming noise in the house (detached ) which is getting worse,it is mainly in the upstairs front bedroom where the airing cupboard is with the copper tank.I have turned off the mains electric but it still persists,it is more apparant late at night although some nights it does not occur at all.

I have had the ballcocks changed n the loft tanks but it still persists,I have also checked the floats in both toilets and they are okay.

I think the problem is definately some where in the pipes but I cannot pinpoint exactly where.

Any ideas please as it is driving me mad !!.