Noises from storage heater?

Three days ago my electric storage heater started making a rumbling noise, then this changed to a loud hum which went on continuously till I turned off the pump. I have had a plumber to look at it. He changed the valve and checked the noise with the day and night storage heaters turned off for a day each. The noise stopped being continual. But now every ten mins the storage heater gives a loud juddering grumbling sound for about 10 seconds. The plumber thinks this is because the pump has come to the end of its life and needs replacing. Do you think a faulty pump could be the cause?