on my set up with the tubs and trays... I have dealt with it 2 different ways.
First set up: I was dumping the water out a single 1.5" pvc pipe right in the middle of the tray. I also used PVC flex tube (from bulk head to the tray top), which made no noise as well.
I found if you put a 90 degree PVC fitting inside the tray on the drain pipe, it helped kill some noise turnong teh corner right before it hit the filter pads.
Also, I have for my first layer in the filter, a pond sponge filter I got at home depot. It is 1" thick and I needed to trim it a little to fit the tray. Then I have a layer of blue mat sold at pet smart in a large bundle. Then I have a few layer of polyfil > (flat layer of pillow stuffing), which I buy in bulk. the polyfil will stick really bad to the sponge filter and clog it, so the blue mat just acts as a seperator. with all this padding, the noise is hardly a trickle.
My second tray is all pot scrubbies. if you drill the holes for the first tray inset from the sides a bit, it will keep the water only hitting the pot scrubbers and keep noise down. Last is the bottom of the tray. I keep the bottom 3" of it in the water that is sitting in the tub, so there is no splashing.
This set up kept the noise down to where you only hear it when you are right in front of it.
Later changes for even more silence, I had my drain pipe into the top tray converted to a spray bar I built out of PVC, which is zip tied to the top, then glued to the top plastic of the tray to seal it. It is basically a U shape. the water comes in a T fitting, and goes left and right to 90 degrees, then forward. The forward section has all the holes drilled in it. I drilled a lot of holes, then another set of smaller holes incase there is a large surge that needs to get down the pipe.
One other things I had done when I first set it up, long before when it was loud. The back of my tank stand could not get right against the wall due the the base boards. I installed a piece of styrofoam along the back of the stand to keep the noise from escaping out the back.
