Pulsacoil III knocking sound

Hello,
My Pulsacoil III has started to make a slight knocking sound when on i.e. when activated by the Economy 7 electricity supply. The water supply is hot and the top tank is full (and hot). Both green lights are on. I've looked everywhere but I cannot locate any red lights. The Economy 7 electricity supply is turned on for three hours at night and again in the late afternoon. During the 'on' periods, the Pulsacoil III seems to turn on and off as if heating the water for 10-15 mins then pausing before heating again. It's during these 'heating' periods that the knocking sound is evident, and I would describe it as a low, dull knocking, almost one knock every two seconds and slightly irregular.
Does anyone have any advice how I can rectify this please?
Thank you,
Tim

Check the rod stats are working ok.
If not could be an immersion heater starting to go.
Or unit could be sludged up.
Mike
MGC/H2O
Norfolk

Thank you for your reply Mike, I appreciate it. Do you know how I can check the rod stats and check if the unit is sludged up - or is this a job for a professional?

If you search on here those items mentioned.
You may be able to check yourself.
But if unsure get some one in.
Mike

Thanks

We have come across similar problems, it was sludge found in the system that was making that noise. Go with what Mike said. Also problems start to increase when people don't use their system correctly. I recommend you request this free self click guide via this webpage Pulsacoil 3 Spares which may help you understand your system better.