Buzzing Water Cylinder when OFF?

We've noticed this a couple of times now; Our hot water tank in the airing cupboard has started making a loud humming noise...but through the night when there is no hot water or heating on. Very irritating - we can hear it across the hall and it's in my son's bedroom so is disturbing his sleep. Is there anything I can do, or anything it might be? I'd rather not have to call a plumber, but guess I will have to - its a gravity fed system, but is about 15 years old (boiler is newer and was fine when serviced 18 months ago). Any thoughts?