Maxistore Direct Combination Unit (tapping noise during heating)?

Although this heater's name says combination this is an economy 7 water heater that heats automatically during economy 7 night hours.

The heating element switches on and off intermitantly during the heating hours but at about 03:30 - 06:30 there is a tapping sound when the tank is heating.

I've checked to see if any of the pipes are virating and they are fine. The noise seems to be coming from the heat element itself.