Hard plumbing gurgle

2nd iraf - you need a standpipe in your overflow box.

 

If you can't, get a durso in your overflow, you have another option, you could put a "T" on it as in my
really "crude" little drawing below, then run the pipe above the water level and put a strainer cap
with a piece of sponge in it to help muffle the little bit of water flowing noise there will be.



I have a 33 long that came out of a pet shop that was drilled exactly like yours. It was noisy
as heck so I did this and the gurgling went away. There is still a little bit of water noise but at
least it doesn't sound like a toilet flushing anymore!