Noisy Pipes on central heating system

[Long John]

I'm having some rumbling and water trickling noises in radiators on my central heating system. When the hot water clicks on there is no problem and if the central heating clicks in at the same time again there is no noise - but if the central heating doesn't come on and the thermostat is turned up to bring the central system on stream there is noise as mentioned above. Not sure what the cause is. All radiators have been bled - boiler is only 4 years old - pump 6 years old. New hot water tank fitted recently.
Any ideas????/

 

[Aquarius]

It could be a sticking trv or an issue with the motorised zone valve.