Cylinder Head Removal

I have a strange knocking noise. When I turn it over or it is running it makes a knocking noise like something is on top of the piston. When I pull the manifold and the spark plugs, I can only turn the crank pulley so far and it stops turning. I put as much pressure on it as I can but it will not turn over. When I use the starter to crank it, it would turn over. My compression on the cylinder #2 is 120 and 180 on all the others. Cylinders 2 & 5 get to the top of their stroke and that is where the engine will not turn over any more. I used a scope to look at pistons and top side (intake manifold side) of the valves, and don't see anything out of the ordinary. I cannot see the bottom side of the cylinder head where the valves are as I cannot get the scope to look up there.

I guess I am just going to have to pull the heads and see what is going on. The book said to pull the engine to pull the heads off. So, I am just wondering why that is so and if I can pull the heads with the engine in the car or not?

Should I be checking for something else?

Thanks for your time.