Do you want the surge & flush to continue??
The problem is that your return line is too small for the pump flow, so it needs to develop a full siphon to handle the flow, but then that is too much, and the tanks drains down to break the siphon, giving you that flushing sound. <And it might be fun at first, but the flushing will drive you nuts after a while!! When I set up my HOB
overflow, the flushing was so loud I can hear it in the next room ..... Sounds like someone flushing the toilet every 3 minutes ....>
Anyways, your solution one will work, but choking off the pump like that will reduce its life. It is better to put a T in the line, have one line going back to the tank. You put the ball valve on the diverting T & control the flow to your fuge with that.
1. Put in another return line so it can handle the pump flow.
2. Choke off the return line so it is running always in a siphon. You put a ball valve in line & adjust it so that the siphon flow is just balanced with your pump. This will be nearly silent. However, you must have a safety overflow installed (ie another hole & return line higher than the 1st). This is because the 1st line will need priming to work properly, and during priming (done automatically as the water level rises), the water level will go up higher than set. The safety can drain the water out so there is no flood. Also, with the return line choked off, any critter that gets caught in it will result in a flood, and a safety will take care of that as well.
#2 is what I settled on. This is one write up on the system:
BeanAnimal's Bar and Grill - Silent and Fail-Safe Overflow System
He uses 3 standpipes (more redundancy!) The original post at reef cnetral is only use 2 (primary & safety), and that is what I have.