Hydraulic lifters need a bit of pre-load so they will be quiet. One reason that you adjust mechanical lifters with the motor at operating temperature is that, at that time, all metals which will expand when heated have done so. If you adjust them on a cold motor, to the noted spec clearance, then they won't be adjusted the same when the motor is to operating temp. The hydraulics, by comparison, CAN be pre-loaded a bit so everything is quiet.
Remember that the piston rings have a slight gap on their ends. The gaps are supposed to be assembled in a manner such that the gaps aren't all in a line, but are staggered. As you turned the motor over, what you were hearing was the compression escaping through those gaps as the valves were both closed. I'd say what you heard is completely normal. When the engine is running, the compression happens much quicker than when the motor is turned-over by hand.
As for the Chevy hydraulic lifter pre-load I mentioned, if it's not 1.5 turns, then it is 1.0 turns, per the specs I read in my '66 Motor Manual. In any event, the desire is to have the plunger centered in its bore in the lifter, top to bottom. Lesser pre-load lets the plunger ride higher in the bore, which used to lessen "pump-up" at higher rpms. With just 1.2 turn pre-load, the plunger is pretty much at the top of the available position in its bore. Should "pump-up" occur, it won't be much. "Pump-up" happened when the oil pressure inside of the lifter was high enough to move the plunger higher in its bore, against valve spring pressure, to keep the valve from fully seating. Which would not let the engine gain rpm past a certain amount. In effect, this was a "protection" against the engine running faster than "somebody" desired it to, possibly for warranty purposes? FWIW.
Now, there's nothing which says you don't have to run a hydraulic lifter with a pre-load adjustment, but that then tends to defeat the reason that hydraulic lifters are used in the first place. Lack of maintenance/adjustment and valve train sounds.
Enjoy!
CBODY67