That video helps a lot! That reservoir is usually half way full pass that intake tube, submerging it - which would then rid of that sound. Same principle as having the emergency and main drain pipe submerged.
You can also increase the water level in the sump resulting in increasing the water level in the reservoir; however, the water level in the ReefMat will eventually raise once junk builds up.
See where I marked the red line? That reservoir level hasn’t dropped down to the sump water level (blue) yet. Usually stays between blue and yellow (trigger censor) after the first 24 hrs or so.