Loud Banging/Vibrating sound.

A few days ago water in the local area lost pressure due to a fault, dont know if this is in anyway connected to the prob but thought id mention it, the water comp said they found the problem on another estate and its now fixed, all seemed well until 2 days ago when a loud, really loud, banging/vibrating sound started round the house eminating from the bathroom tank area and the pipes under the floor.

I called British Gas as I have central heating/boiler care and they have visited twice now but the problem is still happening and I have no idea what to do. Every now and then I get a loud bang like someone turning a tap on and off quickly.. When the really loud banging sound starts, its that loud the neighbours have complained, the only way i can get it to stop is by turning the cold water tap on and off, though this stops it for a short while it soon starts again.

Whats puzzling me is it happens for no reason, even when central heating is turned off and im not using taps etc... Can anyone please give me any clues as to what may be causing this and any info on how to go about solving the problem.. I can call BG out again but am not sure they are who i need.. maybe a plummer? id like to have an idea what it is first..
Thanks for any help you can offer.

Check all of your ballcocks first...

Toilet cistern, tank etc. Could be a piece of debris caught in the washer preventing it from shutting off completely and causing it to hammer.