My bet would be the Fuel Filler venting.
Not sure if it has a Vent Hole in the Cap or not, or if it vents through a separate Hose.

 

Thank you all, i hope i will have time to check everything on saturday/sunday. @IDoDirt @hamlin6 I followed your advice and checked my exhaust, the "psst" sound it makes is completely different and it has a very specific smell, so that's not the case

 

Has your radiator fluid level gone down any? Is there any evidence of a fluid leak in that general area? Given the area where you mention the sound originating it could potentially be the cap leaking after a certain amount of pressure building up.

 

I'm guessing Coolant, Fuel, or Oil dripping onto a hot Header Pipe wouldn't make a loud enough sound (hiss) to be heard, not while riding at least?