What is the noisiest part in your aquarium?

[Dez]

My return pump, but that's in the basement and the tank is on the main floor. Upstairs the noisiest part is the wavebox.

[Parker]

My screams of frustration, second would be the three darts running the system. Each of them alone are pretty quiet but together they make a quite a loud hum.

[zum14]

Skimmer air intake, sounds like someone sucking the last bit of a milkshake with a straw. Except it never stops.

[ponokareefer]

My noisiest was my Rio 32HF return pump that just died. My Mag 9.5 that I replaced it with is near silent. The noisiest part of my tank now is my skimmer, but it is pretty quiet.

[Bloodasp]

Originally Posted by zum14

Skimmer air intake, sounds like someone sucking the last bit of a milkshake with a straw. Except it never stops.

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.