From the picture the standpipe looks similiar to a Durso. If it is, there should be a hole or valve on the top to adjust the amount of air thru the standpipe, is that where you are hearing the noise?
On my tank (DIY Durso) I kept increasing the hole size until the water level in the overflow was stable, when that balance was reached the sucking noise stopped as well.
If that doesn't work might try connecting a piece of tubing to the hole and remoting that. On my old tank used to get quite a bit of noise from the skimmer air inlet that was on one side of the tank. Tried the MJ muffler (helped a little) but added about 4 feet of 1/4" tubing to go to the other side and that quieted it right down.
|