what causes the sewer to gurgle when it sounds and drains good?

Answer 1

That happens when you have a large volume going into the pipe filling the cavity and air is escaping up through it to the drain.

Answer 2

This is usually caused by a venting problem. Drainage systems need air to operate properly. The air is obtained from the vent piping. If there is no source of air, when a fixture is draining, it will cause a vacuum and suck the water out of traps from other fixtures. This breaks the water seal and allows sewer gas to enter the home.