hissing sound from engine while hand cranking

When I hand crank my 351w I hear a hissing sound. Could this be bad exhaust valves?

I just bought this thing and the PO said he had it driving before letting it sit for 2 years. It needs to be rewired and a new carb before it will even consider running. I'm worried this motor will need to be rebuilt.

Probably just compression going by the rings kinda normal IMO. I'd put some marvel mystery oil or even some ATF in each cylinder let it sit a day or two then crank it up. Really only a compression or leak down test will tell you if the cylinders/valves are good or not. I've done far less to some engines that sat even longer and they've been fine

yup totally normal. i would be concerned if it didn't do that at leas you know it has compression.