OK, this might be obvious to most, but while looking through reviews on various sites for different components, I routinely see negative reviews on components intended for use in rack mount chassis because the reviewer thought it was too loud.
They're usually talking about a fan that was meant for a 1U server chassis. 1U enclosures are very thin and you need good air flow through them to keep the components within recommended temperature ranges.
Fans that do that don't tend to be quiet. Don't even think of putting something with a fan, that was meant for a 1U server chassis in your home gaming system that's right next to your bed or under your boss's desk.
The following video shows a typical small office server room (with pretty bad cable management on the racks) to give you an idea what it sounds like.