Reddit mentions of Noctua NA-SRC7, 4 Pin Low-Noise Adaptor Cables for PC Fans (Black)

Yes and No. they do make 4 pin noise reducers (noctua is the best brand, IMHO) https://www.amazon.com/dp/B00KG3KP1U/ref=cm_sw_r_cp_dp_T1_UvKCzbJM2JV6D

However it is a trade off between running the fans quietly, as opposed to making sure the server is running as cool as possible. I've managed to get my supermicro 36 bay server to run in the 30 degree Celsius range for the overall system temperature, but the hard drives generate a lot of heat and I am about to revisit the issue, as I am going to try using what is supposed to be a much quieter fan in it a SuperMicro FAN-0044L4 model fan. I hope to be able to remove the fan speed reducer adapters from my backplane and use these instead of the fans that came in my chassis.

Perhaps an inline resistor.

I used one of these 12V to 7V adapters from Noctua in my H210. Significantly decreased the sound level of my case fan.

I know Noctua sells low noise adapter cables or ultra low noise adapter cables with some of their fans, which (I would guess) do exactly that.

Noctua sells the LNA as standalone product but you can probably get something cheaper:

https://www.amazon.com/Noctua-NA-SRC7-Accessory-Low-Noise-Adaptors/dp/B00KG3KP1U?keywords=low+noise+adapter&qid=1540843342&s=Electronics&sr=1-4&ref=sr_1_4

Is it fan noise or different noises like HDD? Are your fans directly connected to PSU? If so, they'll always be at 100% speed. You could buy a low noise adapter to plug between the PSU and fan so it slows down the speed.
If plugged into the motherboard, you can probably change the fan speeds from BIOS.

Well at one time you could've just used these things

https://www.quietpc.com/rc56
https://www.quietpc.com/rc100

Now it looks like they're discontinued, though I have no idea why and disagree with it. They're very useful to slow down fans that are running using the PSU directly.

Now I did find these https://www.amazon.com/Noctua-NA-SRC10-Accessory-Low-Noise-Adaptors/dp/B00KG3KELQ https://www.amazon.com/Noctua-NA-SRC7-Accessory-Low-Noise-Adaptors/dp/B00KG3KP1U/ https://www.amazon.com/Noctua-NA-SRC6-Accessory-Low-Noise-Adaptors/dp/B011EY3GME/

But I don't know the actual resister being used though I believe the spec sheet on Noctua website will reveal more info.

Note: Never use one Low-Noise Adaptor to slow down multiple fans

Noctua seems to have a few adapters around, not sure which you'd want

You could always add a resistor to the fan power wire if you're up for some soldering or just buy an adapter like this one from Noctua.

okay, so i have the 3 pin fan, which means it requires the external PWM signal. the fan also comes with a 2 pin adapter, that takes it down to just red (+) and black (-) but that doesnt really matter because its the same thing i did, i just soldered the black and red wires and left the yellow one alone. its kind of interesting though because these fans come with the low noise adapter which changes their RPM, so the speed is variable, but i suppose it doesnt work with varying voltage like what my previous blower fan did.

I have a pair of these you can have for the cost of shipping, if you are still looking

Edit: hmmm looking at your amazon link, it appears they are 3-pin adapters. Mine are 4-pin :(

Edit 2: mine are these

You can grab a 3-pack of low-noise adapters for $8. They're basically just expensive resistors with a wire and pins, but they work really well. Obviously you can't get variable fan speeds using these, but you can at least reduce the RPM easily if you so desire.