+1 :eyebrows:

The gurgling sound coming from my overflow to sump pipe....which will be fixed on the next upgrade project coming soon.

noise
 

actually ,it's the pump(gen x pcx 40-75 lpm) for my bullet 2-but it's not very loud

There are a bunch of diy stuff that you can make to silence the air intake.
On that note my skimmer pump is the noisiest part of my system. Next is my return pump which couldn't be heard if the sump cabinet was closed.

I second this one, There is just a slight hum from my tank and it is probably from the 4 pumps running my 2 skimmers.

on my 40gal reef the loudest part is the skimmer for sure, its the injector thats loud (remora HOB skimmer) I tried putting foam in there but doesnt seem to help much.

as for my 35gal with sump I would say its the water rushing into the sump, next time I would do 2 or 3 smaller pipes rather than one big one..

I voted main main pump because of it's constant humming but also the overflow box which makes a slight trickling sound.

For me, running 3 x 400 mh I have 2 x 80 cfm fans cooling the fixture, although they are only 35 db, if they turn on after I get up, it is almost unbearable. I use a bubble king skimmer which I consider second noisiest in the tank, follow by the anti siphon holes on tank water return pipe. I also use a red dragon 1200g return pump. The absolute best investment I ever made, ultra quite, super performer, very happy with it.

the Air sucking in from the Dual 1262 eheims from the Deltec AP 702

For me it's the air intake on my BK 180 mini.

I had that too - until I used a piece of styrofoam to float the motor so it doesn't vibrate against the bottom of the tank. And the additional advantage is that if I do hear it humming on the bottom, I know it's time to top up the ATO reservoir.