Suspect the hissing noise you refer to is the exhaust gas venting outboard after the combustion has created the expansion of gas that spins the motor up.
Unless of course you saw a couple of cylinders and hose leading to the engines, but I can't for the life of me imagine how they could rig that up.
Resorting to BEagle-speak, it would normally sound like:
WEEEEEE PPPSSSSHSHSHSHSHSHSHSHShhhhh.......
Tarnished