Is the gurgle like a 'burp' i.e. bubble releasing or like a falls
within the pipe? I am trying to figure how the flow gets to sound like
a gurgle.

How big a diameter does the closet flange have? Our returns are 4"
toilet flanges exiting 55 gal barrels. They handle a large flow. The
water flows down a standing pipe if we use the bypass we built into the
falls. The sound is the water falling about 4' vertical inside a pipe.
It does not compete with the waterfall, but is a distinct sound.

Do you know what water motion is causing your sound?

Jim.
Cam wrote:
I have a submersible pump feeding an upflow filter. It makes an
annoying noise. The pump itself is silent and the sound of the water
flowing into the pond is pleasant. The problem is the sound of the
water flowing out of the filter. It makes a rushing/gurgling sound. The
water leaves the filter through a closet flange in the side of a
barrel. Can anyone offer a tip on how to silence it? It drowns out the
waterfall.

Cam