Inconsistent fan speed at idle

I've recently put (my first!) custom loop into my system and have been seeing and hearing some strange things from my system fans. The way I am currently configured I have my pump connected to my mobo CPU fan header. The rest of my fans are connected to the fan hub that came with my case, which is then plugged into one of my other mobo fan headers. I have two fans (Gentle Typhoon AP-15) on my rad and two stock case fans, a 140mm in the back and a 180mm up front. It should be noted that I got the AP-15s used, so I do not know how much use they have seen prior to this.

I am running them in SpeedFan (4.52) I have the fans set to SmartFan IV and linked up to CPU temp with a pretty standard curve. However, the fans have a very noticeable change in speed even if I am not doing anything particularly taxing on my system. They will rev up, then slow back down fairly quickly. It seemed pretty random to me, so i left my computer to idle at the desktop for an hour or two and let SpeedFan chart it. I attached the graph. Red is my pump, green is the fan, which in my case is the hub that's controlling all four of my fans.

Anyone have any advice on how to smooth this out?


My loop is only a CPU loop currently. Honestly made it for fun and silence more so than performance.

Relevant hardware:
i5-6600k + EKWB Supremacy EVO
ASUS Z170M PLUS
PWM DDC3.2
2x Gentle Typhoon AP-15 fans + EKWB PE240
Phanteks Enthoo Evolv mATX (non-TG)

I set the "curve" to be flat vs temp with the fan speed just high enough to do the job. So say 40% @ 20 deg, 40% @ 30 deg, 40% @ 50 deg, 40% @ 70 deg, then let it go on up to 100% at 85 deg. This keeps the fan speed constant when little temp spike come and go, but if it starts running legit hotter the fans will ramp up. As long as your entire loop doesn't just keep getting hotter and hotter (dial fan speed up across the board), it'll stay at a constant noise level and not constantly ramp up and down.

Check your bios for fan spool up times and spool down times.

Unfortunately, this is the way any watercooled system will behave if you control your radiator fans off your CPU temp. The CPU gets hot quickly when under sudden load, and even at "idle" your OS is periodically doing things that cause bursts of CPU activity.

Ideally you want to control your fans based on the difference between your coolant temperature and ambient temperature. That will usually require some form of dedicated fan controller with it's own sensor inputs; for a simple system like yours I'd recommend an AquaComputer Quadro.

If your mobo has a temperature sensor header, you can control your fans based only on coolant temperature without buying dedicated hardware. It's not as good as the above method because it doesn't consider ambient temperature, but since most PCs are in a climate controlled place the ambient can be assumed.

Either method will help your fans spin up slowly and only when they need to.

dullard and van are right. the way I get around this is I look at the temp spikes and set the fan increase to only start a couple degrees above the highest spike. as they said the normal cpu spikes are what cause it and this will prevent them from coming on until the cpu is actually under a sustained load, which raises the cpu temp that couple degrees above the spikes.
so say in normal use it spikes to 60 at the most. set a flat line of 25-40%* with it set to start ramping up at 65 and max at 85.
*you can play with the fan speed to get the temp/noise ratio you like

something like this:

yes its afterburner but you should get the idea.