Even when you have the A9x14 running out of the case, at higher rpm it's always audible. Everything is RPM dependent.
If you mount it under the motherboards, at low rpm (+/- 600 ~ 800 rpm), it will be hardly audible.
If you run it at full speed, it will be loud.
I had it installed in the normal 92mm fan position with the rubbers and it was still loud when cranking up the rpm.
If you just connect it to a fan header on the motherboard and use a fan curve, it will be silent most of the time.
I use it on the 545LC and on normal tasks, it's dead silent. Once you stess the CPU or start gaming, you will certainly hear it, but at the point you start gaming, you GPU fans will start spinning and the PSU fun (in case of a corsair/silverstone psu) will start spinning as well and that one noctua fan will be the least of your concerns.
Just think about the fact the PCI riser cable will likely block alot of airflow.