CPU Fan weirdness

[Rakk]

So recently, my CPU fan seems to keep ramping up in speed - sometimes for a few seconds, other times upto about 30-40 secs, I found out about this cos when it does so it makes a lot of noise, and yes it's definitely the CPU fan.

The PC details:
Case COOLERMASTER CM690 MKII ADVANCED CASE
Processor (CPU) Intel® Core™i7 Quad Core Processor i7-4770 (3.4GHz)
Motherboard ASUS® Z87-K: ATX, USB2.0, USB3.0, SATA6GB/S, XFIRE
Memory (RAM) 8GB KINGSTON HYPER-X GENESIS DUAL-DDR3 1600MHz
Graphics Card 2GB NVIDIA GEFORCE GTX 770
1st Hard Disk 240GB KINGSTON HYPERX 3K SSD, SATA 6 Gb/s (upto 555MB/sR | 510MB/sW)
Processor Cooling Super Quiet 22dBA Triple Copper Heatpipe Intel CPU Cooler.
Said machine is now about 2.5 years old

Now the really odd thing is that when the fan ramps up, the CPU tends to be staying very stable and doesn't seem hot (it has happened when the CPU temp is at 26C which is blantently not hot for a PC), so I don't think it's a temperature problem.
Some evidence from SpeedFan attached:
These two were from when I'd just turned on the machine one morning last week (first is the CPU temp - yes it was rising but then it had only been on for a few mins and wasn't exactly hot, second is the CPU fan):


And these two from earlier this morning once the PC had been on for a while (first is the CPU temp at a particularly stable 33C, second is the CPU fan):


Yes, I've checked for viruses and malware, found nothing, sometime I can hear it doing it on the lock screen - before I've even logged into my machine.

So anyone any clues what it could be, or what else I can look at to discover what's going on, or whether it's something I can kind of ignore?

 

[jerpers]

I had similar problems with the Titan cooler in my old machine. I used ASUS Fanxpert and had no joy getting it back under control. I have no idea what the problem was caused by but I removed it, cleaned it up, blew in the fan plug (because that s what you do isn't it? haha,) repasted/plugged in and all worked fine.

 

[Rakk]

Thanks jerpers, glad to know it's not just me that has seen it happen, I was hoping to get away with not having to remove the whole thing - its quite large and unwieldy - and well I'm a bit of a static magnet (I do have one of those wrist thingies to wear to discharge static, but I still prefer to stay away if possible).
Though I may see if I can find where its plugged into the mobo , unplug it, spray some compressed air on it and replug it in as a first step.