You can clip the Noctua in using the oem black plastic bracket.  I wouldn't recommend drilling the metal chassis unless you remove the mobo and electronic components first. But there's no need to do that. 

Lol same

@r72019 @Anonymous Thanks so much - didn't realize it was a bracket and not the fan assembly itself.  Appreciate the help and pointer! (and no, wouldn't drill / Dremel / case modify with stuff in it)

If you would please post your impressions after fan replacement.  So I'm gathering corsair 120 pro for top fan and Noctua is recommended for front fan?

Thanks,  I've been very happy with the performance of the R11 so far, but if I can optimize the system all the better. If you would, I'm interested in your thoughts after you get both fans installed.

@DrkHwk I've replaced the front fan with the Corsair ML120 Pro and will be tackling the top fan replacement this weekend.  I decided to use identical fans after reading about issues with the Noctua in the top fan position.  I'm happy with the build quality of the Corsair.  Hopefully the fan noise goes down a bit afterwards.  You may find this thread on Reddit interesting as well!

Blue Book

Corsair ML120 Pro (no LED, no RGB) 

Noctua NF-A12x25 PWM 

There is some optionality associated with the lower front intake fan.

According to Digital Etron's video, the 120mm case fan must be 1,700rpm or faster to prevent a failure to boot. Is he referring to the top fan only?

If so, I'd love to try out some fancy (Arctic P12 PWM) fan to replace the front one. :3

I wanted to follow-up to this post as today I finally had some time to replace the front fan, radiator fan, and re-pasted the CPU.  Bottom line - very quiet at the Balance profile, and much reduced at the Performance profile.  The temps are reduced by about 10 C, which could be the paste (most likely).  The fans spin much slower than the factory fans.

Bottom line - worth it to me.  Since I was in there I re-pasted CPU so that might have helped.

Links to some pictures

I had to replace my front fan when I went from a 2080ti to the only off the shelf 3090 I found that fits. I had to go with a 1/2" slim fan. Its the only one I could find that would fit. its running fine but also causing the annoying BIOS error. I'm thinking about running a Y cable and just setting the factory fan in there somewhere.