Our installer measured each room and the existing rads, and estimated the heat loss from walls, ceilings and windows, based on the age of the house. Then proposed which rads needed replacing based on the lower flow temps. We ended up replacing 10 out of 12 rads. Kitchen has UFH so left as it is. One of the rads was replaced because we didn’t think the old one was enough, based on last winter with the old oil based system.
Very pleased with our installer, who just do heat pumps and solar, but only used for hot water so far (Eco Installer and Service, just outside Ely, Cambs). As to noise, if you’re outside you need to be within about 2 metres to hear it. Inside nothing.