sounds like poor flow rate, or boiler stat. Do all rads and hot water heat up. If so pump may be ok. Turn stat down to min, If still noisey you may have to change boiler stat.
mike alex, November 2008
Check if there is water in system bleed upstairs rad / water in header tank
Possible boiler stat us, is the boiler getting to hot and blowing out of vent into header tank
