Noise as in a sloshing water noise? Like the pipe is too wide, maybe?

Also, how would I attch a Hofer to the 3" overflow? Just friction fit it inside?

The noise is caused by Air, plain and simple. You need to eliminate the excess air and to do that you need a gurgle buster or durso. To use either of these you need an Overflow. Adding a T like Newguy said I don't think will work as it will just suck the air from your 3" opening. A gurgle buster is essentially the same as a durso, just you don't need as much space in the overflow. You 'might' be able to friction fit a pvc pipe with holes drilled in it, inside your 3" pipe and slide it up and down until your flowrates mesh. It's risky as the top of the gurgle buster pipe would probably be above your tank maximum level. In the event of something like a snail getting sucked in, your tank will overflow as there is no chance of a syphon being created with the rising water level.

I'm above average with cichlids, but feel like a complete idiot here.

So you're saying that I need a gurgle buster, but it will probably overflow my tank at some point?

How about this: instead of the 3" overflow that is on top of the elbow leading through the glass.....what about another elbow facing down into the water?

Due to your hole being quite low in the glass you need an overflow box. Otherwise modifying your drain so that it's completely submerged will cause your tank level to drop greatly. By doing this you can use either a durso in a similar fashion to what 'newguy' said, or you can use a gurgle buster as you will be able to have it lower than your tank level so that it won't flood over. This is the reason that all drilled tanks have overflows, if they didn't need them they wouldn't have them. You could also try cutting your flowrate through the sump way back and adding an additional powerhead to the display to make up for the loss of flow. The less water flow through the drain the less the noise 'should' be.

With the elbow, I was thinking the water would have to flow up the elbow and over into the drain.....I guess it would still drop just as much, like a regular siphon only larger....