How can I stop my Sime Halstead Trio Whining?

Hello everybody. I have a Sime Halstead Trio boiler. When I run the hot water it fires up fine but just before it modulates it starts to whine its head off! I've replaced the DHW thermistor but to no avail. Has anyone come across this before and can offer any other ideas? I drained down the system to fit new radiators which is when the problem manifested itself but is this just a coincidence? The central heating is fine. Thanking you in anticipation.