System is sealed fully pumped powered by Stanley Brandon oil fired cooker/boiler. Airing cupboard with cylinder and valves etc separated from head end of bath by stud partition. Pump seems OK. After running a bath the header tank in the loft finishes filling after about 2 minutes, then a loud throbbing noise (like a single cylinder diesel on tick over) starts from the airing cupboard and continues for about five minutes then fades out. There is no loss of pressure in the system. Any ideas will be appreciated. Thanks