You can hear them alright. Everyone is different but I can hear them while sitting at my desk with the computer on the floor. I use speed fan and have them set to 40% they cycle up when needed.
Qe fans on 12v are noticeable to me unless i start gaming then the game noise drowns out fan noise. 7v i can't hear them at all. You can get better fans for the money in my opinion for 12v use, 7v many fans perform the exact same.