I'm putting my engine back together after a blown piston. With the head mounted and without the cams installed I was able to turn the flywheel by hand to insert the locking pin. After thinking about it, I realized the spark plugs were in and I can hear air flow with the valves closed. Is this normal? It's not easy to turn, but I wonder what I should hear and how difficult it should be to turn by hand since I haven't done this before.

 

that is normal. the air will escape through the ring specially on cold spin. also, because you're doing it way slower than a started would, that's also expected to happen.