That's not normal. First - you're sure it's coming from your overflow and not a power supply or pump? Are all of your drain lines completely independent and not merged together at any point?The sound is almost like a whistle sound
I think mine was stuck in the overflow tube for at least a day once. I noticed the level went up in the overflow and there was more water going down the partial than normal. It had been like that from about the time I went to bed one night until late the next afternoon. Finally decided to open the valve on the full siphon - let things kinda equalize then readjust. After that is when I noticed the fish in the filter sock. I'm pretty sure he was in the overflow, probably stuck against the valve until I opened it all the way.My internal overflow is 60″ long and has several 1/2″ slits routed out. The only fish I’ve had make it to the sump via the water slide is a small wrasse.