A&M Aggie 04'
I'm assuming you have a tank with a hole drilled in the upper back side of the tank, with this bulkhead fitting installed.
Bulkhead probably has an 90 degree elbow or some fitting turned down with flexible drain hose or PVC piping leading to your sump.
Correct me if I'm off track here.
If I had this set up, I would not want the tankwater level to cover the overflow/bulkhead completely. When this happens, you've turned this overflow into a situation where you are siphoning the tank water to the sump. I believe you need an air vent for the water to drain naturally by gravity. In other words, you want your tankwater level to be somewhere in the middle of the hole/bulkhead fitting, allowing both water and air to enter this opening.
When you have the drain hose under the waterlevel in the sump, I agree with Glenn above.
Air ... as it is allowed to enter the overflow opening is getting trapped in the hose. This air gurgles and burps up the hose making the noise you hear, but allows the water to flow faster to the sump, keeping the tankwater level down to somewhere more in the middle of the overflow opening ( bulkhead ).
When you lift the end of the drain hose out from under the sump water level .... this allows the air to not be entrapped in the drain hose, but sounds like it does not drain as fast, allowing the tankwater level to rise up over the bulkhead opening ... and once the tankwater level rises up completely "over" the opening of the bulkhead ..... you have a siphoning condition.
Only water is entering the hole now.
Couple things you can try.
See if you can move the open end of the drain hose in your sump, into a postion where it discharges the water "horizontally" across the surface of the sump water level. I had to install a 45 degree elbow on the end of my drain hose, with a short 6 inch piece of PVC pipe on the end of this elbow. This allowed the water to shoot out sort of across the top of the sump water level, and still allowed air to get out as well. This reduced the noise I was hearing a lot.
The other thing you can try is installing a PVC ball valve on the outlet of your return pump.
You can throttle back the flow from the pump up to your tank, play with this a little, and see if that helps.
These PVC parts can be purchased from Home Depot or Lowes or most any decent hardware store's plumbing department, and shouldn't cost you more than $5-$10 bucks, depending on the sizes.
Naturally, these are just two things that worked for me. I'm sure there are other equally or better ways to solve your noisy situation, and maybe some others here will offer their ideas or solutions.
Lastly, to help out in understanding your exact set up ...
What size tank?
What size sump?
What size opening/bulkhead?
How many openings/bulkheads?
What size drain hose(s)?
What model return pump?
How much vertical distance from the return pump up to the spraybar/exit of the return hose?
