Please silence my groaning system

[Alexcunliffe]

I had a Telford Tempest Mains Pressure Domestic Hot Water System installed 3 years ago, It works fine in terms of producing hot water BUT if we use the hot water whilst the boiler is heating the system or just after when the water must be at its hottest, we get the most horrendous groaning from the cylinder area which can last for 4 or 5 seconds. If we are using the hot water when the boiler is not working, there is no problem. Anyone got any ideas? The plumber who fitted the system has tried lots of things (pressure reducing valves etc) and I am constantly trying to equalise the pressure in the expansion tank to the mains pressure coming into the house. I have even consulted Telford but all to no avail.