The water is siphoning out of the filter and makes a flushing sound as
it rushes out in pulses. I'm going to try lowering the whole filter to
reduce the height differnce.

AHH - does your return line have a "low spot" in it - i.e. - does it have a
gentle, gradual fall to where it discharges? - if there is a sag in the
line, that could cause a gurgling sound, and if the sag (or low spot) in the
return line is enough, it will overflow your barrel - the return line must
either be level or have a down slope with no sags
Gale :~)