Not sure what is going on here, but my shower is leaking behind the wall but going down the drain.

I know this because the drain became completely clogged and water started to rise in the tub. Also the shower is on the second floor and I have seen now signs of water damage below.

Any ideas?

tyhe shower should not be connected to the drain in any way. it creates a dangerous cross connection. someone who knows plumbing should look at it.

I would find it more likely that something else is leaking and both drains are obstructed downstream and backing up to the lowest point-- the shower. There is probably a sink dripping or toilet running occasionally that is providing the water to compound your problem. I suggest getting the drain cleared first from a dependable drain cleaning company which warranties their work, then find the leak before something overflows and causes water damage.

Drain is now clear but you can hear the leak. And can find it leaking anywhere from below?!?!

If you can hear the leak, and it was going into the tub, it is behind the wall next to the valve, probably spraying on the wall and running down to the lip of the tub. If you don't have access behind, make access; if you can't make access, remove the trim plate and take a look with a flashlight and inspection mirror.