any suggestions to reduce noise??
Noise in the pipe is created by air bubbles trying to float, while water is trying to "sink". Pvc Is incredibly noisy, but is the best choice for a reef tank.
It looks like you have 2 drain lines from your photo.
I keep 2 drain lines on all of my tanks. Both of my lines discharge under the water level in the sump. My goal is to regulate the "siphon" line with a valve, and in doing so causes it to remove almost all of the water volume the sump return pump delivers. The second line is only to catch the very least amount of "overflow". (Google "Herbie Overflow")
I have never had major issues. Restarting the pumps can cause the "siphon" line to have air in it for a bit, but it will work out in a short time.
If it is noise, listen to see which pipe is making the racket. If it is the secondary line, you may have luck changing the size of the hole in the air vent at the top of the tank. Smaller may be better. I often use automotive plastic vacuum barbed fittings to reduce the size of the hole. It may look silly, but I had to test a few sizes until I got it right.
Good luck, and I hope this helps.
I wanted to add this as an Edit, because it just crossed my brain....Is your drain pipes totally vertical without any bends or slope?
