Fan Control on Rendering PC

Unfortunately you are trying to fight reality. Fans blow air, and air flow removes heat. Higher temperatures inside your case cause the system to request higher fan speed to achieve higher air flow. Of course, those higher speeds and air flow also mean generating more fan noise. You can NOT get more heat removal without getting more fan noise.

You have already done the most important thing to reduce fan noise in your system. You chose Noctua fans, which have a reputation for durability and higher air flow delivery at lower noise levels (for any particular fan speed) than most competitor products.

Most mobos have TWO different cooling control systems, although they operate in a very similar fashion. In each case the control loop has a temperature sensor to measure actual temperature of the item being cooled, a setpoint target for that temperature, and a means of manipulating the speed of the fan(s) plugged into that header so that the airflow they generate can bring the measured temperature to target. Although we all talk about "fan speed control", these systems really are TEMPERATURE control systems, and the only manipulate fan speed to achieve their goals. Most mobos have one system for cooling the CPU, and its basis is a temperature sensor built into the CPU chip itself. In your case, with a dual-CPU chip design, I suspect there are two of these systems, one for each chip, and each controls only the fan plugged into the FAN7/CPU1 or FAN8/CPU2 fan header. Then the mobo has at least one separate temperature control system for cooling the entire case interior, based on a different temperature sensor built into the mobo, and guiding the headers used for system fans. That one system may be governing all of the FAN1 through FAN6 headers - can't really be sure because the mobo manual is not specific at all. On p. 4-11, although it lists several temperature sensors, it also says the BIOS really only accesses and displays for you the two CPU temps and one "System Temperature", so maybe that latter is used for all those six case fan headers.

The case fan temperature control system will have some internal table of how to change the fan speeds according to what is measured for the System Temperature. Details are not provided. It may operate on the basis that one temp range calls one fan speed, and the next range up calls for a faster speed, etc., but there is no indication how many such steps there are in its strategy. It does give a clue (MAYBE) in telling us it looks at CPU temps in terms of three levels below an alarm upper limit. Since the documentation also describes four possible control options, it may actually use four slightly different tables to convert a measured temperature into a fan speed setting. Full Speed just uses full speed all the time, I guess. The other three appear to use progressively less fan speed (and airflow, and noise generation) for any given measured temperature.

I recommend you remove the "Low-Moise Adapters". That name sounds enticing. But what these items really do is add a resistor into the power feed to each fan so that it runs more slowly than the mobo header intended. What that can do is fight the automatic control system. The system sets a fan speed according to measured temperature, but the LNA added in makes the fan under-perform, so the system keeps on heating up. That forces the control system to raise the fan speed, destroying the dream of slower quieter fan. If you remove the LNA's and just concentrate on choosing which Speed Option to use in BIOS, that will get you closer to what you need. Those LNA'a really are intended for use only by people who are not using the mobo fan headers and the automatic control systems at all. They are used when the fans are connected directly to a PSU power output that can exercise no control, and thus the LNA is the only way to reduce fan speed.

I suggest also that you look a little more widely at what you expect. The case fans are supposed to respond to temperature inside the case on the mobo, whereas the CPU fans respond to different temperatures inside teach CPU chip. Your post compares fan performance only to the CPU temperatures, and ignores other temperature readings. For example, although the GPU systems have their own cooling controls, they discharge heat into the case interior, raising the System Temperature. This would require raising the speed of case ventilation fans even if the CPU chips' internal temperatures were not rising.

You may not have been aware that there are two or three fan speed (really, temperature) control systems operating. Your post does not indicate which fan(s) are speeding up at times - it appears you believe they all do the same thing in response solely to CPU temperatures. Re-examine more closely which fans are doing what, and which temperatures are involved, during these times. That may help you to configure the cooling systems better. One thing to look for is the possibility of making different choices for the CPU cooling and case cooling options. p. 4-11 of the manual does not make this clear. But maybe you can choose different options for the case fans versus the two CPU fans.