My toilet has developed a loud thrumming noise - rather like a distant helicopter - whilst not flushing. During the actual flush, and the refill, the noise stops. Turning on a nearby tap also makes the noise stop, as does closing the inline valve on the supply pipe to isolate the toilet refill. But otherwise, this noise comes and goes more or less at random.
I've tried fiddling with the refill valve, which is a cheap looking torbeck. This seems to have changed the frequency and volume of the noise but not made it go away.
I am assuming the problem is the refill mechanism, rather than anything else in the toilet (or indeed elsewhere in the water system). To minimise the number of trips to screwfix, should I try changing the diaphragm in the valve first, or just go straight to replacing the whole torbeck valve? Or something else?