I just ordered a pair of 120mm gentle typhoons (1450rpm) from NCIX (on sale 2 for $20). Unfortunately, they suffer from the same problem as the S-flexes I got a year ago. At full speed they make pleasant whooshing noise (though at 1450rpm, a bit loud), but any sort of undervolting results in the whooshing disappearing and in its place a terrible buzzing noise. I will grant that both are excellent at moving air. I also have a sleeve bearing slipstream that ticks at lower RPMs.

However, they fail to hit a satisfactory noise floor with reduced voltage, and my fans of choice continue to be Noctua. Of course, I'm using non-comparable fans (S12B and NF-R8), but they are capable of pushing some form of air while sounding much quieter with no rattling or ticking or otherwise annoying sounds. These are perfect for situations where fully passive is simply impossible, and these fans add enough circulation to bring temperatures to an acceptable level.

What are you using to undervolt your fans ?

Are you using a PWM controller?

I hope this doesn't make me look stupid, but I've been using either the Noctua LNA/ULNA that come with most Noctua fans. Additionally I have a fan controller in a 5.25" bay that presumably uses variable resistors.

Is there a chance this is causing the problem? The GT fans are 3 pin so I don't think they are PWM?

I have a GT that I undervolt with a Scythe Kaze Master 5.25" bay fan controller, and it seems perfectly happy. I also have used such controllers with 92mm Scythe Kama Flex fans, and 120mm Scythe Slipstream fans, all without buzzing.

OP - I felt the same way, there was a buzzing/ticking/pulsing with all my fans, Noctua, Slipstream and the FT02 180mm case fans when controlled by the touch screen NZXT fan controller - no PWM here.
Then today I decided to run them all off the motherboard fan headers - then near silence! All of them running around 600-700rpm under auto control

Huge difference...

Now there was something I thought was puzzling - when I used the fan controller to control the case fans, using the cable that comes with Sliptreams, that allows for RPM monitoring to a motherboard header - the RPM were bouncing all over the place, going up to 3000+ and back down to where they should be.
Ontop of that the speed settings for the FT02 case fans, which run in parallel with whatever your controlling them from, had no effect (they work when using the motherboard headers)
So that kinda clued me that something was not right....

It seems like the fans were pulsing on/off max speed in split seconds and thus buzzing!
I just thought it weird that all these fans have claims of 6-15db etc, which SPCR reviews and others here have claimed as really quiet, and yet didn't seem to jive with my reaility - now faith has been restored

So not all fan controllers are doing it the right way. I believe my old Zalman fan controller in my other old case, does not have this issue, I'll see when I swap it in

After repurposing the fan, the issue seems to have lessened to the point where it is acceptable.

As a soft-mounted intake (with filter), it produced the buzz.

As a hard-mounted exhaust, it sounded ok.

As a soft-mounted heat sink fan, it produced the buzz.

Weird...

fuzzymath10 --

If you read my review of the GTs, you would know these fans do not like to be run at reduced speed. You must select the fan w/ default 12V speed that's right for you. Voltage reduction causes tonal noise to appear.

My GT doesn't have this problem.

What my "research" (i.e. anecdotal evidence) shows is that fans seem to be the most heterogenous parts in the IT industry. Even with the best fans out there, you have (fantasy stat) 1 in 3 chance that you get one that hums, buzzes, whines, ticks or something else.

When you run a 120mm fan at 1400 rpm you will hear it. Some more than others of course but it's likely too noisy for most
At 1000rpm they in general become fairly quiet, but not silent, rather not that annoying to many folks
At around 700-750 rpm I feel you have a very quiet fan and reasonable airflow.
Below 500rpm very quiet but not shifting a lot of air

I tend to aim for 700rpm odd around there use what you want fan control etc, ULN adaptors no problems
I'm talking in general now, there are variations of course but that's the general vibe I've had over the years.

Interesting update. While these fans fail horribly as intake fans (my original configuration) and as blowers on HSF, they are pretty decent at exhaust, and blowing away from HSF.

I use them to push air out the top/upper side in both my cases which happen to have passively cooled GPUs with moderate TDPs (5750 and 4830, ~90-100W). I have laid out the cases to draw air through the open PCIE slots, past the GPU, and rise up past the CPU and out through the exhaust.

The GTs are very, very good at forcing air to be drawn past the GPUs. My GPU temps are considerably lower with no more weird instability, and the fans are decently quiet. The only drawback is that in this configuration, the CPUs run hotter since the fresh air gets heated by the GPU before passing the CPU, but because I undervolt and leave stock clocks, the temps are acceptable.

I'm curious if it's the fan or the configuration which is making it work so well; I want to try my slipstream and see how much the temps change.

I have a nzxt sentry 2 since 3 good years, tested with many fans/brands and i haven't noticed "buzzing noise" unless it's a high speed (+1200 rpm) they make noise( no buzz though)

For your gpu there are encased fans that you connect on a pci bus they exhaust the air behind, i've never tried but it's worth a shot i believe
edit:
i've found the link :
http://www.xoxide.com/slotcooler2.html