when i flush my toilet and it starts to refill a deep quite loud humming noise occurs. The toilet is a close coupled lever handled type with syphonic action and mains fed into cistern from bottom entry. the noise occurs at start of refill and ends before cistern has finished filling. however it doesnt seem to make the noise whenever you flush wait for it to fill, and then flush again. Its more when it hasnt been flushed for a while it is noisy. Could it need the ball valve split and checked for dirt, or a new one. Any suggestions appreciated.