Right, you are all going to think this is my imagination and i'm going crazy but here goes:
74 CB550, has electronic ignition and a solid state regulator / rectifier all new. New bog standard lead acid battery.
Bike starts on the button, rides fine.
If I don't use it for say 2 to 3 weeks, when I put the key in the ignition and turn it on (just on, no attempt to start it), the bike makes a brief hiss then wont do it again.
Bike fires up perfectly, idles fine, rides fine.
The hiss noise will not come back for a least a couple of weeks if the bike is not used.
Hiss is nothing to do with the fuel tank, petrol tap is off when it happens.
Just wondered if there was some kind of valve in the battery maybe to release pressure?
It sounds like it comes from below the seat so i'm thinking battery. Battery is fine, holds a charge.
Please, no Snake comments