Noisy Rads only when both CH and HW

[Bulleid]

I have an open system with 3 speed Keston boiler and 3 port valve (also boiler bypass). The HW system is left over from a Potterton thermo-syphon hence large 28mm pipes. Silent in CH mode and HW modes separately. The HW is valved so that when both modes are selected it restricts flow to cylinder so house gets some heat. For the last five years the boiler finds some combination of flow/temperature that generates a sound like a ball bearing rattling in the supply pipe to the HW system and sounds through all the adjacent rads. Have change 3 port valve... no difference.

Pulling hair out... anybody help? (I don't think a ball bearing is present!!!)