Because you said you heard a hissing and compression is low on one cylinder. I have to suspect a blown head gasket. As said earlier typically blows between two cylinders, but compression says it did not. It is possible for one cylinder to leak outside the head or into the lifter valley.
Is there any anti-freeze in the
oil? Check this before starting engine again. With engine running is there any air bubbles going into radiator?
I never tried this, but perhaps some soapy water around the head gasket and then fire it up might confirm a compression leak from a blown gasket.
I agree with whoever said a leak down test. You can listen for a valve leaking into the intake, or the exhaust, or into crankcase.