It could be worn crank seals, particularly the one on the left hand side (generator side)
Try removing the cover over the generator ..start the engine and spray a jet of WD40 (with straw attached) through the slots in the rotor aiming at the crank seal. If the engine changes tone as you do this, it will confirm the problem if it's there.
If when you take the cover off you see wetness and smell petrol, this alone will be confirmation enough.