Boiler noise in hot water system only

[warrington]

It is gravity system. Boiler was making noise. I cleared the system, fitted total filter. And chemicals added. The problem in central heating side solved no noise at all, but when asking for hot water it starts noise again.
pump works ok, 3 mid way valve ok, I am bit confused really.
I ll be greatfull for any advice.

 

[SimonJohns]

Sounds like its a modulation problem or over gassing. If range rated. Turn it down slightly