Again, it's hard to say, as everybody's ability to hear it different, especially depending on how much loud music you listen to etc. But I would say you'll be able to hear it if you tune into it, but you'd most likely become habituated (accustomed) to it very quickly. It'll be less noticable if it's under a desk or something, or if there's other noise from either the pc (power supply often being the biggest) or something else in the room, but 80mm fans are conventionally more noticable than their 120mm alternatives. That's why people are opting for those where possible, and people are even buying mounts to convert a space for a 80mm fan into one for a 120mm fan.