the safety valve is passing slightly,could be a bit of grit on the valve seat or maybe the spring is getting weak.Try opening the boiler casing and look for the valve,it might have a manual override(sometimes a lever,sometimes a cap that you can turn,once you know it has a manual discharge try opening valve by hand to see if you clear any grit,if not then its easier just to replace valve,they aren't too expensive and quite easy to replace,remember to drain system first.
steffdaplum, March 2009
