I don't have an answer, but I think I have the same problem. A reasonably new 330+ boiler frequently (but not always) starts with a cabinet rattling "Hrrummmp" that we hear throughout the house and even outside. We had a BG service man out (of course it was not doing its worst then), and he checked settings and pressures and said everything was fine. I've been trying to descern a pattern to no avail, but it is really getting to us and probably time for another call. This time I have been making some recordings which I will play for the service man through the stereo.
Richard de Oldphart
February 2010