Drain Water Noise Quieted after Inserting Vinyl Tubing into Drain

[Harry Muscle]

I have a Glass Holes overflow (Durso equivalent) in my new tank. The overflow itself is nice and quiet. The noise has been coming from the actual drain pipe. It's a gushing water sound. Sort of like someone upstairs is draining a bathtub. Most likely this is because the sump is in the basement so the water has a long way to travel and either slushes around on its way down or maybe picks up too much speed to stay quiet. I've played around with a few things to quiet things down and surprisingly what worked best so far is inserting two 7/16" OD vinyl tubes into the 1-1/2" drain pipe. They start about 6" from the tank bulkhead inside the drain pipe and end just inside the sump sticking out of the drain pipe slightly. This takes the noise from being annoying to quite and gentle. Now I'm curious as to why this actually worked. Has anyone ever tried something like this before? These vinyl tubes aren't acting as additional vents cause they start inside the drain pipe. The two main things that come to mind is that they reduce the size of the drain pipe slightly (still more surface area than a 1-1/4" pipe). They also add 50+% more surface for the water to cling to. Any practical or scientific insight is greatly appreciated.

Thanks,
Harry