Noctua NH-D15 chromax.Black, Dual-Tower CPU Cooler (140mm, Black)
Corsair 4000D
I have a 3080 TI as well but I'm not ready to touch any of it as I paid an arm and a leg for it... I know I'd get a better one for about half the price these days but I don't want to touch it...
and no overclocking.. all default
Alright, I actually have nearly direct experience with an almost identical setup, and know exactly what's going on here. That system had an i9-9900K, NH-D15S, a 3080 Ti (EVGA FTW3 Ultra) > 3090 (EVGA Kingpin Hybrid) > 3090 FE, all in a Corsair iCUE 4000X RGB.
The problem is simply this: you're dumping upwards of 400-500W of heat directly into the compact interior volume of that midtower. More specifically, the 3080 Ti is sitting right under that big air cooler and feeding it significantly pre-heated air, causing the heat dissipation efficiency to drop and the fan to spool up to 100% to try and compensate.
First and foremost though, I would strongly advise against a CLC in the 4000D. No room for a top mount, so you're left with front-mount 240/280/360 intaking and feeding CPU-preheated air to the GPU instead. Unless you've got one of the Strix 3080 Ti variants that GPU is almost certainly making more noise than anything else in your system under gaming loads.
The 4000D comes with just a pair of basic 120mm fans. To get this case capable of properly ventilating that much heat you're going to want to fill every available slot with a suitable (with maybe the exception of the top rear slot, as it gets choked by the sheer volume of the D15. If keeping the 4000D is your plan, you'll want to pick up the
replacement airflow front panel for it ($15 from Corsair).
Next step is planning airflow properly. For the front what you want is 3x 120mm high flow fans. If you want to make a bit of an investment, you'd be hard pressed to find a better all-around option than Noctua's own A12x25 PWM, though it can be tough to stomach the sheer cost there. Corsair's new AF120 Elites are another great option here, in particular because the airguides in the frame help maintain a strong windtunnel effect pushing front-to-back. Both have enough pressure to give you adequate airflow through the magnetic filter screen, however removing that will certainly increase airflow at the cost of quick dust buildup. For the opposite end of the price spectrum, a pair of 3-packs of Thermalright's TL-C12C-S 120mm fans can be had for literally less than a one of the above mentioned options. If used on intake I'd definitely remove the front filter and use the airflow front to minimize flow losses due to the filter restriction.
For the rest - a 120/140mm fan mounted to the front-most top slot as an
intake (blowing down in front of the CPU cooler) will increase turbulence to better ensure the D15 is getting cool fresh air rather than the GPU's air coming up from below. I used an NF-P14 Redux here. And finally a 120mm exhaust on the rear - I used an NF-S12B Redux).
After that, it's really just down to digging into fan curve tuning to optimize for silence. What often happens is that the reactivity of tower coolers means broader fluctuations and short-term spikes in core temps under regular use, but most mobo default CPU curves will end up either cranking it to 100% as soon as the chip is doing basically anything, or that unpleasant roller coaster ramping up and down constantly with each change. I prefer a curve that integrates a large flat temp range at 40-50% for anything from idle temps up to around 60C. Then a ramp up to 80% for 80C. If your fan control solution has the ability to set the temperature hysteresis or ramp delay, use that to add some delay time to smooth out sudden short temp spikes that drop back down right away.