Elusive fan noise

[rogor]

I have replaced all my case fans with 5 Pabst 8421NGL 80mm. They are very quite fans but when the computer is on, I hear a high buzzing noise. My cpu fan is a Nexus D12SL-12 120mm. My psu has only one 80mm (not pabst) but it has a rheostat which I keep at the lowest rpm. My vid card is fanless.

I can hear some turbulence and I understand that, but I'm trying to track down the buzzing noise. The thing is, I've stopped all the fans individually, including the cpu fan and the psu fan; I even powered up without the HD connected. The buzzing is always there.

 

[Talcite]

Buzzing can also be a sign of cheap components. I'm sure you've heard lights, PSUs, or other electronic components buzzing when they are turned on.

 

[Zepper]

Is it coming thru your speakers or the mobo buzzer/speaker?

.bh.

 

[sbuckler]

Is it your psu - particularly as they age they can start to buzz?

 

[GalvanizedYankee]

The way I read it, you have 5 same type Pabst all hooked up to 12V.
Maybe they are harmonizing...I'm not kidding. Paired fans will do this. That's why running one at 11V really helps.

 

[piasabird]

Some times the inside of a case is like an Echo Chamber or a Guitar, and it amplifies the sound. Having some vibration absorbing material between the fan and the case sometimes helps. Also the pattern of holes in the case can slow down the air and cause fans to work harder, because more air is being forced through the existing space. Some people cut out a square where their vent holes are on the back and put a grill on the outside.

 

[rogor]

It was the vibration of the 842N1GL fans causing it. And I think having 5 of them compounded the problem. Here's what I did:

- disconnected all but 3 80mm and undervolted one of them to 8v.

- replaced the 80mm in the psu with one of the Papst fans because the original fan wasn't the greatest. It did have a rheostat though which I soldered to the Pabst so now my psu is much quiter. I know you're not supposed to do that but I have one 80mm blowing in from the back of the psu also.

- attached the top port hole fan with 3 layers of double sided spongy tape to isolate it from the case instead of using screws

- the fan blowing into the psu from the back was already isolated with 1 layer of double sided spongy tape, might add more someday.

- I undervolted the 120mm cpu fan to 8v


Overall the machine is much quiter now, but there is still a faint vibration sound from the 80mm's (high frequency) and slight bit of wind noise (lower frequency so it doesn't annoy me as much). At lease now I know what the source of that sound was.

I think I've decided that 80mm fans aren't the best option for a quiet case. they are just too small and therefore require too much rpm's to move x amout of air. And those papst's are about as good an 80mm as you can get but I still heard the vibration. An undervolted 120mm will push more air with lower rpm and therefore less vibration. I guess isolation from the case is a must also, regardless of size or fan quality. Thanks for the replies.