If hissing is heard at the carburetor air filter, it is the intake valve. If it is heard at the crankcase breather tube, it is going past the piston rings. Valves or valve guides could be worn or it could be broken piston rings. It could cost many bucks for this job. Engine compartment should be inspected for excessive oil leaks, condition of exhaust system, hoses, wiring, etc.
Small leaks are common.
Continentals are kind of like a Harley Davidson, if there is no oil leaking out of it, it probably doesn't have any in it. Check engine mount for bent tubes which indicate hard landing on nose gear. Check hours since major or top overhaul time on mags, generator, starter, exhaust gaskets for leaking, loose muffler or carb heat muff. Connections to exhaust pipes are insufficient to retard pendulum effect of outlet pipe. Engine should start within 3 or 4 revolutions.