When I have my hot water and central heating on at the same time
I get an awful noise in my radiators.
When I turn the hotwater off the noise stops.
I have a standard gas boiler and separate hot water tank set up.
My plumber doesn't know what causes this.
Can anyone help?

It sounds like the heating system is "backflowing" slightly when the inlet water pressure decreases with usage, and you are experiencing cavitation within the radiators. Check to assure the system is equipped with some type of backflow preventer, and that it is functioning correctly.

Could be air in the system. Has any of the heating system been modified recently ?