Crackling noise from Pulsacoil III

During the night time heating cycle the tank makes a continuous crackling/popping sound. The water tank level is OK and I fitted a new thermostat which did not cure the noise. The noise stops if I switch off the off peak mains supply. It also stops if I switch off the pump mains supply. Any ideas what it might be? David

I've now checked that the noise also occurs when the top up immersion is switched on and the noise stops when I switch off the supply to the pump/control board (with the immersion still on). The red LED on the control board is flashing twice a second, but there was a five minute period straight after I removed the cover when I observed it flashing at a faster rate. The hot water temperature out of the taps is very hot. I wonder if the noise is water boiling at the surface of the immersion element, it does seem to emanate from that area. Any ideas would be welcome. David

I have now had it repaired by Midland Boiler Care. I was very pleased with their service. The crackling was due to a build up of calcium on the two immersion elements, so it crackled like a kettle that needs descaling. I had somewhat discounted this (hence the reason for my original post above) because the product marketing says the immersion elements remain in the same water which has scale inhibiter in it, and I have never had to top it up very much with new water.

However I did see for myself the scale on the old elements when they were removed and it was quite thick. Midland Boiler Care explained to me that the rate of scaling increases with temperature, and I am in a hard water area. I am still left with the opinion though that Gledhill's product marketing gives an over optimistic picture.