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.