Our central heating makes a horrible howling noise from the area where the pump is, and this problem can be worse if the boiler dial is set any higher than minimum. As soon as the boiler fires up as part of its on - off cycle, the noise diminishes considerably. Once the boiler stops burning, the noise gradually starts again. The pump and boiler are probably about 15 years old. Any advice (especially from gasman) welcome.
Gasman will no doubt get there too - but sounds to me like the bearing in the pump have gone - especially if 15 years old. If so, IMHO buy a Grundfos and accept no other they are miles better than other brands. If you have lockshield valves on each side of the pump (most installations do), its a relative simple DIY job if you fancy it because you can isolate the pump and get it off without draining down the system. Buy either a Grundfos UPE 15-60 about �50 or an Alpha Plus about �80.