Here's a few things you can try if you want;
Look at the 5 foot length of hose. As it leaves the overflow bulkhead, does it droop down, then raise back up to connect to the trickle filter cover plate ? Assuming it does droop somewhat, see if you can eliminate the droop so the hose gradually slants towards the wet/dry. Support this hose with your hands, raising it up a little and listen to what happens. If this quiets the noise, then support this hose with a make shift support.
You may want to try a different return pump. What pump are you using now. Do you have a ball valve installed between the pump and the return line to the tank. Having this valve allows you to alter the flowrate to the tank - sometimes this can balance things out a little - and reduce the noise.
I don't recommend installing valves in the draining hoses or pipes of an overflow drain - although some people do this - and have reported good results. Just don't restrict the flow too much - and you may find this approach works well in your case.
Increasing or decreasing the hose diameter is another option, but again I'd be careful in making reductions.
Increasing or decreasing the hose length is one more option. Some people have extra length of hose, and coil it around, or make turns in the hose. I don't like doing this, it seemed to make mine louder. But again - all depends of each person's set up.
I prefer as straight a shot as possible from the overflow to the sump, with little restrictions, turns or low spots. This seems to allow the air an unrestricted path to vent at both ends of the hose.
Although this is not the best example, have you ever taken a one gallon milk jug full of water and try to pour it out fast.
If it's full, and you turn it upside down real fast, water begins gurgling out and you see big bubbles as the jug needs to vent to empty. So much water pours out, then you see a big gulp of air ( like in an office water cooler ).
If you tilt the milk jug as you pour it, keeping the mouth of the jug open so air can pass along with the water - the jug seems to empty a lot smoother, and it just sort of flows out.
If while you have this jug upside down, you stabbed a hole in the bottom of the jug ( now the top ) with a sharp tool, air rushes in and the water flow right on out no matter what angle you have it tilted.
Now imagine a piece of pipe extending out of the milk jug. You can see what may happen as this pipe is extended out farther from the jug.
This is sort of what can happen with our overflows and the hose/piping leading to the sump area. Sort of.
Anyways - gotta take the dog to the vet now.
Good luck - let us know how you make out
