The European GTS does not make this noise, as it has no evap system. Removing the evap system will have a similar effect of stopping the noise.
Here is a picture of the evap system cannister which has been removed from a GTS
and some other links to the symptom.
My scooter "farts"
Why is my GT200 mooing? o_O
GTS Noise after running
My GTS was farting today...
Weird moaning sound coming from my Vespa?! (audio clip)
The article on how to disable the evap system:
EVAP Hose Removal
LX150 Complete EVAP System Removal
Evaporation System: canister replacement
MP3 250: Removal of the evap canister and roll over valve