Hyundai generator issue - any ideas?

[Monkeysmate]

If any of you mechanicy types have any ideas why my genny has started struggling I'd be grateful. When it's running, and I apply a load to it that it could handle very happily a couple of weeks back, the engine strains, the revs drop, and it trips. All the while it is running there is a background hissing noise, which wasn't there before, and when the genny stops, the hissing stops with one final "spit". Any suggestions would be gratefully received. Ta