Monster 1200 R 'sighing' periodically'!

[Khanivore]

My bike periodically 'sighs'! It happens even if I haven't ridden it for several days and comes from the fuel filler cap area. What's causing it and is it a problem? Video of the noise, caught just after it started, here:

https://youtu.be/PPv7a1nv1Po

[slob]

Blocked breather pipe perhaps?

[mickj]

It's probably the overpressure in the tank being released, quite common on warm days as the petrol gets warm and expands, nothing to worry about