Is it part of the AIR system? Its the big one just after the the bend, its got a thick rubber hose held on with a plastic clamp. Seems like when I removed my AIR pump, it left the hole open, so I just plugged it up, but I'm wondering if I somehow forgot to plug something important back in.

 

No that should be for the AIR pump. Just leave it plugged if you removed the AIR system.

 

Yep that is simply for the air system.....removed mine this last summer and I just plugged up the hole.