Phyllis and Jim wrote:

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.

I used to have a veggie filter (15 gallon aquarium) mounted above my 200
gallon aquarium, and had plenty of opportunity to watch this phenomenon.
If you have a really big pipe, so that there's always air space, but the
water can pass it by, you don't get gurgling. If you have a really small
pipe, or the outlet is always below water level, you won't have gurgling,
but have to worry about the possibility of overflow. Probably, if you can
keep the outlet pipe vertical (at least from an elbow exiting the filter)
you won't get the gurgling.

What happens is that, as the water level in the filter rises, air gets
caught in the outlet pipe. As the water level (and pressure) gets higher
and higher, the bubble gets pushed further into the pipe, until it escapes
out of either end (causing the gurgling), then water rushes out of the pipe
until the level in the filter is once again below the top of the outlet
pipe - and more air gets into the outlet.
--
derek