mines started doing the same, when was the radiator last flushed through? should be every 2 years at least, and also top ups ? if the expansion bottle is a nasty rusty colour, you could try that , thats my plan any way - nice new clear expansion bottle from reno is 24 quid, looks good! Haynes has a section on it in if you fancy a go, had a look the other day, need a hose pipe, water and hands th esize of a two year old!! good luck!!