Every time my heating system comes on, for either the water or heating, the pump makes a loud rattling noise for about 5 secs before it quietens down and then the system works fine with no noise. The pump is new (Peggler terrier TC 5), I've flushed out the system and bled all the rads, pump etc. The pump is running on setting 2. I've tried setting 1, but not all the radiators heat up although the rattle is reduced. Setting 3 causes overrun. It sounds like there is air in the system when it first starts up but I don't know where it is coming from. It can't be doing the pump any good. System uses a 3 port valve and a Glow-worm Spacesaver II boiler. TRVs are fitted to rads in bedrooms only. Anyone know what the possible problem and solution may be 
