SOLVED Random noise changes with fixed speed fans

[MakoShark]

I've started noticing this today after installing a new disk drive yesterday and re-arranging most of the cables in my case. Every few seconds the airflow noise seems to increase for a few seconds and then decrease again. The change seems quite random but is very abrupt. As far as I can tell, it seems to be coming from the area around my front intake fan.

It only happens when I have the case side on so it must be related to turbulence or pressure inside the case. It is a Zalman Z11 Plus case with stock rear and top exhaust fans, the stock front fan moved to the bottom intake, and a Cooler Master Sickleflow on the front intake (where the change in noise seems to originate). All case fans are running at full speed.

Any suggestions on how to solve this? I would connect the Sickleflow to the motherboard to try running it slower but the cable is too short so it has to be connected to a molex.

 

[MakoShark]

Just tried connecting one of the 'chipmunk cheek' fans to see if it made any difference. Seems to have dramatically reduced the frequency, duration, and noise level of the noise changes and there is quite a lot of air coming out through the cheek. Might try connecting the other one as well in a bit.

Might it be possible that a build up and release of pressure in the hard drive bay area (3 out of 5 HDDs installed so not much space for clean airflow) was causing the change in noise/fan speed and the exhaust of air through the cheek is helping reduce the pressure build up?

 

[Conumdrum]

No, no build up and release. A PC case isn't a ballon.

If you connect each fan one at a time you might just find a bad fan, or the one causing vibrations.

Procees of elimination methinks.

 

[ehume]

When I run fan tests with my i7 860 OC'd to 4GHz, the PSU comes on runs for a bit, then stops. It's possible you are running into the same phenomenon.

With a Z11 you can vastly improved your HD cooling by removing the plastic clip on the downstream side of the HD cage. You can hold the HD's in place with the included thumbscrews.

Remove this clip:



I covered your case in my review here.

 

[MakoShark]

No, no build up and release. A PC case isn't a ballon.

If you connect each fan one at a time you might just find a bad fan, or the one causing vibrations.

Procees of elimination methinks.

I did think a build up of pressure (or turbulence) was very unlikely but it was the only thing I could think of to explain why it would only happen if the case side was on. It could just be that the case side is emphasising the noise though.

When I run fan tests with my i7 860 OC'd to 4GHz, the PSU comes on runs for a bit, then stops. It's possible you are running into the same phenomenon.

With a Z11 you can vastly improved your HD cooling by removing the plastic clip on the downstream side of the HD cage. You can hold the HD's in place with the included thumbscrews.

Remove this clip:

I covered your case in my review here.

I have read your review before and have already removed the plastic clip. Thanks for the review.

I will have to check my PSU to see if it's fan is starting and stopping. I wouldn't have thought that was normal behaviour though. Or do you just mean revving the fan up for a bit and then going back to idle speed?

I'll try testing each fan individually to see if one is the cause.

 

[MakoShark]

I made a lead extension so I could connect the Sickleflow to the motherboard and monitor the speed.

I spent a while just watching the fan speeds in the bios and experimenting with different things. With the case side on, the Sickleflow stayed at about 1925RPM most of the time but every 10-15 seconds, it went up to about 1950RPM for a few seconds before going back down to about 1925RPM. The airflow noise increased and decreased at exactly the same times as the fan. All the other fans were more consistent without any sudden speed changes. With the case side off the same thing happened with the same speeds, just less often for some reason.

I wouldn't have thought that a 25-30RPM variation is unusual for a 2000RPM fan but it's surprising that such a small change creates such a noticeable change in noise. The suddenness of the change also seems odd.

The volume of the airflow noise isn't what I find annoying, I just find the random change in noise distracting. I have now set the fan to slightly slower in the BIOS and it seems fine.