Hi Just after some of your thoughts on this puzzle please , I have a strange rattling noise when ch is on , through big bore pipes that send the HW from boiler to rest of house , boiler is ground level, pump is in airing cupboard upstairs. Rattling sound like debris flying around the pipes , there is a vertical run of pipe from gnd level to upstairs. A new boiler was fitted a year ago , works well. New pump , new controller Plumber drained out old water and added the additives to comply with the guarantee for the new fix. Any ideas what this strange sound could be ?