Is the level in the overflow cycling up and down? If it is, the air hole may be too small. Mine plugged with salt a while ago and it was cycling and gurgling again. When I cleared the air hole, the gurgling disappeared.
I have installed a 90 facing downwards in my similarly sized overflow, fully submerged and the overflow is completely silent. Sounds like we have a similar setup.
Good luck, my tank is in my bedroom and has to be quiet.
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