A standard standpipe is not meant to be silent. It continually sucks in air at the full siphon level, causing the gurgling sound. There are numerous ways to try and make it quieter. One is the Durso pipe - essentially you put an upside down U at the top of the standard standpipe, with an air hole drilled at the top of the U. The Size of the air hole regulates the siphon level, thus reducing air suction and noise. The problem with Durso is that it really limits flow capacity. The rimmed tanks from Marineland and Seapora use Durso.
The most popular silencing method currently is the Herbie overflow. As with the Pro Star you have two standard standpipes. One is about 6” lower than the other. The higher one is the emergency overflow and sets the water level. The lower one is the main overflow. The main overflow never makes noise since it is fully submerged. To ensure full submersion of the main drain, a gate valve is used to restrict the flow on the main drain until the water level reaches the emergency overflow such that a trickle of water goes down that. Herbie requires fine tuning, but is completely silent once dialled in. It also achieves higher flow rates than Durso. Red Sea, etc. use Herbie. Many people who own rimmed tanks convert their overflows to Herbie by converting the Durso pipe to a standard standpipe and using It as an emergency overflow and cutting the return pipe 6 inches below the water level and using that as the main overflow.
I’d expect the best thing to do with the Pro Star would be to cut one of the standpipes by 6 inches, and add a gate valve to it in the sump to convert the overflow into Herbie. It is not clear to me how exactly to do this though since I have read that the standpipes and the return pipe are glued in. If that is the case, perhaps the return pipe T at the top can be removed and then one of the standpipes can be cut with a Dremel? I am not sure how much space there is in the overflow box. Figuring out how to do this is in fact what is delaying my purchase. I want to make sure I can convert it to Herbie before I buy it.
PS: As for the lower side holes in the overflow box, I read that those were removed from V2. Perhaps
@flyfisher2 has V2 and
@mindme has V1?