Just removed and reinstalled a soak tub. I'm using a Watts trip lever drain asssembly (used to have to reach into the tub to pull the plug). Anywho... Now, when draining, it gurgles. I don't remember if it did before. When I put my hand over the drain (to slow down the drain flow) the gurgle stops. I tried to remove the trip lever piece but that didn't help. What causes gurgling or air suck in a tub drain?