If I hear this noise when starting the bike, is it a problem? It only happens sometimes... What will the cause be?
EDIT: Ok I see that it is too much pressure, but is this something to be concerned about? Why is there only too much pressure 'sometimes'?
When starting it is more likely that the noise is caused by a lack of oil on some moving parts. Personally, I don't worry about it as the oil pump quickly gets the oil where it should be. There is also a rather complex description of the engine starting to turn while the valves are closed, so making compression, or going down with throttle bodies closed, so sucking a "vacuum". I suppose it could also cause the knock, but although I have read the piece in this phenomena, it still eludes my level of understanding.
Anyway, engines do knock a tad when starting, so generally not worrysome.