It is not a noise or vibration from pc case - it coming from the pump. Yesterday I've taken it from pc and held it in hand - same ultrasonic noise even on low rpm's.
It's driving me crazy already. Apparently it may still resonate with the case though. 2000 RPM - deadly silent. Any higher rpms and you get this high tone.
yes that's how they sound.
Depending on the mount type and the case they will resonate at some speeds and not at others. mine was noisy around 55, 66 and 75% PWM but in between it was quiet. there's no rule, it really varies depending on the pump mount and the case.
You may want to try to increase the speed slowly and listen if you have silent spots at higher speeds.