Strange noise when turning on ignition

[paultsmith]

am having an intermittent noise sometimes when I switch the ignition of my bike on.
It is an 81 XS400.
The noise sounds like a large puff or release of air. It only happens sometimes and occurs about a second or too after I turn the key to run. It happens before I start the bike. It also seems to happen when the bike has been sitting and moved around without turning it or starting it for hours.

Any thoughts on what could be causing this?

I thought it may be the battery breather venting or perhaps the engine breather releasing some pressure, but am not sure why it would happen all of a sudden once it gets the 12V of power.

May also be the carb breather vent or gas tank venting????
It happens every once and a while and I cant seem to pinpoint the problem as it happens pretty quick

Anybody experience something like this before?

 

[souperman]

Could a plug be firing in a wet cylinder? I recall my CB500T doing something like this once in a while.

 

[rockcitycafe]

+1

 

[paultsmith]

Ok, that makes sense.
I was having a problem with only 1 cylinder firing for quite some time. Both cylinders would only kick in at 4K RPM, so I have been idling it and revving it quite a bit, but have not driven it since last November.
My plugs/cylinders were quite fouled with gas from doing this. My problem turned out to be I had one bad spark plug, so I replaced those and now its good, but I need a good ride to clear everything out.