[Hairytank]

Just to be certain where the noise is coming from...according to your post it sounds like your noise is coming from your sump and not the overflow..is this correct?
ie: when you stick your head in the stand over the sump..the sound is coming from there and not from above...?

Some possible solutions if it is the water entering the sump that is causing the noise.
If the end of the plumbing for the water to enter into the sump is submerged too much then there will be backpressure and it is more difficult for the air to get out until pressure builds up and then large bubbles will be forced out relieving the pressure until it builds again. This will cause fluctuations in your water as once the built up bubbles escape then the water flows better, lowering the water level.
A couple of solutions for this one, angle the pvc entering the sump and have it no more than 1-2" below the surface. If the plumbing is perpendicular (90 degree angle) to the water then it is harder for the air to be released. I would also add a filter sock to the plumbing entering the sump and this will break the bubbles up some and help reduce noise and bubbles in the sump. Besides a filter sock if maintained properly will polish your water nicely.