After hooking up my newly purchased NXZT Sentry 2 yesterday and experimenting with the fan speeds etc i've noticed that the top 2 fans (akasa amber 120mm fastened with rubber fan mounts) seem to make more noise when undervolted compared to when under full speed. Is this normal? it doesnt seem to make a difference with any of the other fans I've got so I'm confused. The fans are configured as exhaust in a CM690 case. I'm wondering if the undervolting is causing some vibrations somewhere perhaps?