Gurgling Overflow - any ideas?

[fruitwerks]

HI All!

I got an old 70 gal with a DAS h39 overflow, we decided it was time to do our first marine tank.

I have a baffled 20gal under-tank for a sump... I did a quick PVC overflow following this design - http://www.aquariumlife.net/projects/diy-overflow/120.asp and I am fairly happy with my initial setup and test, although it gurgles about every 10-15 seconds.

Anyone know what the issue is?

- I'll start another thread for the tank setup itself

Thanks!

 

[fruitwerks]

Forgot to mention, the overflow is 3/4in PVC and the return pump does 215gph and has a 1/2in hose.