noisey overflow
 

I have the same tank I did an external durso drilled hole in cap to 1/4 it was noisey so used silcone to make smaller about 1/8. It had 2in sch 40 pvc pipe on it when I bought in but I used 2in sch 40 pvc for durso and then went to flexible 2in pipe for ponds. It is quiet and Im flowing about 900gph. Much like dkcrx's but not at angle.

That looks like the cause of your problem, your overflow is becoming a syphon due to the number of reductions you have made with adapters.

I would try throttling back your return pump.......I have HOB over flow that claims to be rated for 1200 gph, and I tried everything to make it quite.
The only thing that worked for me was to back off the return pump to about half.

Here is a great article on reducing the noise from your overflow

http://www.wetwebmedia.com/SystemPIX...Oneinchart.htm

Hope that helps

OK, I changed the overflow and line to straight 1.5" pipe all the way down to the sump and if anything it is noisier now! It is a swirling rushing water sound. Not flushing and not even really gurgling. The sound of water rushing down pipes. Like if somebody upstairs flushes a toilet and you hear the water rushing down the pipe.

The T at the bulkhead and pipe are angled down at roughly 45 degrees and I used only 45 degree elbows to get it down into the sump so there are no hard turns for the water to make.

I've also tried increasing or decreasing the flow from the return pump. Doesn't seem to help.

This thing is driving me nuts as I can't get it quiet. Help please!!!!

Does the drain pipe go under the surface of the water in the sump?

It does go under the water in the sump.

Is it possible to raise it above the surface?

Juuuuust above the surface so the air that has been introduced up top can escape. Having the pipe come down at an angle and then putting a 90 elbow on the end at the bottom about halfway submerged worked best when I ran a Durso/Stockman.

Also, did you do something about changing the size of the air hole?

I actually have a reverse Durso at the bottom so air can escape. No back pressure in the line and it is not flushing or siphoning at all. It's not an air gurgling sound but a constant loud sound of the water flowing/rushing down the pipe. I do have an adjustable air valve in the cap at the top but changing that does nothing for the noise. It only changes the depth of water in the overflow box.

I have pretty much come to the conclusion that trying to run a single external Durso overflow from a bulkhead on the back of the tank near the top is extremely difficult and a poor choice. The best way to run overflows drilled at the top back of a tank is the herbie style. I am going to see if I can come up with a clever external herbie run off one bulkhead.

It's rather difficult to solve a problem if we don't have all the facts, do you have pics?