The water returns via 1 1/2 inch hose, the waterfall is fed by a
separate pump.

I don't have the ability to increase the size of the opening without
rebuildind the filter and replacing the hose. I tried jamming a piece
of filter material into the opening and the noise disappeared! the
water level in the barrell rose a little but now it all runs
silent.Thanks for the ideas.
Hi Cam - I have no idea what is causing the "noise" - I have built 40 - 50
upflow filters for customers and use a 2" return bulkhead and line and have
yet to hear about the noise you are talking about
the piece of filter material you jammed into the return line is an
accident waiting to happen as the filter material will load up and slow the
flow until your barrel overflows and pumps your pond down
Gale :~)