Is it mounted correctly? Usually motherboard flex from a cpu cooler is due to tightening things too much or mounting incorrectly in some way like using the wrong mounting hardware or the wrong holes/slots on the mounts.
Buildapc
I don't think it's possible to over tighten a noctua heatsink. They have hard stops and you're supposed to tighten it to those stops.
https://noctua.at/pub/media/proslider/banner_lga1700_home_3.jpg
Ignore that it's the intel one, but you can't really overtighten this mount. I just thought, OP you have the backplate installed right?
I could be overtightening. I will try to be careful not to.
Edit: It appears you were right. Thank you.
Awesome, glad you got it working!
Review the manual again and redo installation. The cooler is not properly installed.
Something else is wrong with your system if an NH-D15 is causing the system to not boot. The flex from the heatsink shouldn't be perceptible.
Are you missing a bunch of motherboard standoffs or something? Is your CPU not socketed fully somehow?
I never knew overtightening was a real issue until it happened to me. I was going to send back my motherboard until I saw someone suggest to loosen the screws of my heatsink.
That actually worked.
Fabricate a support bracket or turn the PC on its side.