Im especially thinking of the Noctua fans but i've heard pwm fans give a ticking motor noise from the pulses is this right?

Thanks!

According to Noctua this will not be a problem with their PWM range because they use circuitry aimed to eliminate the possibility of ticking. See http://www.noctua.at/main.php?show=ne_fd1_pwm_ic. If your motherboard supports PWM fan control then certainly consider going for PWM fans, provided that the BIOS and/or supplementary software gives sufficient control so that you can reduce fan speeds to low levels. The Noctua PWM fans have a range nominally starting at 300 rpm so very low fan speeds at idle are certainly a possibility.

It's not that simple.

There are two main kinds of fans: 3-pin and 4-pin. The 4-pin fans are generally referred to as PWM; they use the fourth pin to control the speed using a PWM signal. 4-pin fans do not click. Since 3-pin fans don't have PWM built in, their speed is controlled by the average voltage of the "+12V" pin. Some of these fans click when controlled by some motherboards.

There are two main kinds of fan control: voltage and PWM. All 4-pin motherboard fan headers use PWM, and a few have the option of using either PWM or voltage control in 3-pin mode. It has become very rare recently for motherboards to support voltage control in 3-pin mode; the controller chip that has come to dominate the market uses a modified PWM scheme.

That's where the clicking comes from: modern 3-pin fan controller chips use PWM rather than analog voltage control to vary the fan speed, but that screws up the speed sensor on a typical 3-pin fan. To compensate, the controller puts out a full-voltage pulse a few times per second, which causes a sudden torque change in the fan, which makes it click, but also makes the speed sensor work correctly.

Bottom line: If your motherboard has enough 4-pin fan headerss for your needs, use 4-pin PWM fans, even though they are a bit more expensive. If instead it has lots of speed-controlled 3-pin fan headers, go ahead and use 3-pin fans, but expect some of them to click. Most brands have a lot of sample variance in the clicking noise, so just buy one or two extra fans and use the quiet ones. A completely different alternative is to use a voltage adjuster such as the Zalman FanMate, which never results in clicking, since it is totally analog.

4 pin fans can tick too, an arctic cooling pwm I was using in my htpc did that. Now fans can tick for other reasons too, but I think it was the working frequency of the pwm circuitry that did that as the ticking went away after I plugged it on another motherboard.

Overall, the main reason for using pwm fans is to take advantage of the automatic speed control features offered by modern motherboards. Like cmthomson mentioned, motherboards that can control 3 pin fans are becoming a rarity. On the other hand, there are some after market fan controllers that can control fan speed according to system temps (scythe makes some that are not too expensive) but they tend to complicate cable management as you`ll need to deal with the cables of the sensors on top of those of the fans.

I have not done much trial and error on this, but SPCR seems to have come to the conclusion that, noisewise, it is much harder to find a quiet 4-pin fan than a 3 pin fan, even frmo the same vendor using basically the same technology, fans, bearing etc on both fans.

Part of the issue here is that you cannot necessarily judge a PWM fan by passing fixed voltages such as 12V, 9V, 7V etc through it, you do need a specific PWM fan testing arrangement of some kind. As far as I am aware, SPCR has not to date had a PWM fan testing rig so relies on results from fixed voltage testing. Doing this is going to be particularly pointless with fans such as Noctua's PWM range where results from testing elsewhere indicate that the fan is less noisy under PWM control than it is under direct voltage control.

I have noticed that. But SPCR has made comments from time to time that must have been based on something. Maybe they will have something so say about this in the upcoming fan review.

With PWM fans a trick I'm currently using is to wire a Fanmate (analogue version) in to the +ve and -ve wires, usually red and black, of the fan by swapping the pins in the plugs around. The 3rd speed reporting wire and 4th PWM speed control wire are left untouched. The advantage of this is I can then alter the voltage the fan is running on which alters it's speed range. For example my TY-140 fans normally run ~700rpm (up to 30%) to 1300rpm (100%) but now run 500 to 1000rpm.

I believe the fan controller that comes with some scythe PWM fans has two modes, one that does exactly this and one where you manually set the fans speed and the PWM control is lost.
See here.

Regarding whether 3 or 4 pin fan is best I would say it's more down to the quality of the fan. Unfortunately motherboards are moving to 4 pin PWM fan control while the best fans are mostly 3 pin. I would expect this to change slowly as more and better 4 pin PWM fans become available.
When I first wanted/needed a PWM fan the only ones that looked worth having were Arctic Cooling AFxx25 PWM models, and I could only get them from Germany!

Seb

I should have been more specific. PWM fans don't make the 5-per-second or so click or tick that 3-pin fans often do with many newer motherboards (as long as they're plugged into a 4-pin PWM header on the motherboard). They can and often do buzz, rattle or otherwise make mechanical (as opposed to air turbulence) noises.

The best example I can offer is Nexus. Their 3-pin fan is so quiet it has served as SPCR's reference fan for many years. I bought one of their 4-pin fans and it made so much racket that I threw it away. It was noisy at all speeds. Yuk.

Currently I'm using Arctic Cooling 4-pin fans on all my 4-pin headers. They are loud at full speed (turbulence, not hub noise), but very quiet at lower speeds, once the software control kicks in during the boot process.