[hwillis]

There is no such thing. You can have quiet fans, but there are no fans that block noise. It is not possible to do so; the best you can do is long baffling. An example would be central air in an apartment building: the vents are very long and do not join each other until they reach a central point, which helps attenuate sound. This is a very inefficient process, and the baffle has to be much longer than the distance you'd have to stand in a free space to not hear anything.

[danielheath]

Depends how much air you need to move.

For one person, a baffle of 10x 1cm sheets of plywood with a 2cm air clearance will do wonders (works out to be approx a 30cm cube with a 30x2cm slot at the front and back).

You can push ample air for one into a studio without any audible sound getting through.

[tgsovlerkhgsel]

There are companies that sell these (mostly to keep homes habitable when a new noise source like an airport runway affects them). I don't know how they work, but I'd assume that they pull the air through baffles and/or noise-absorbing material.

[NickNameNick]

Could you put active noise cancelling into the ductwork?

[hwillis]

sure, but the whole point was not using noise cancellation.