Some have been saying they've been moderately noisey, others say it's nothing to worry about. I'll judge it myself, ordering myself one very soon. My friend says his' isn't making too much noise, just occasionally. Get the slot model (105S), It's really cool, plus you get a digital connector to your sound card (For CD audio).
It's also, I may mention, really easy to RPC1 hack. My bud did it in 10 minutes, no complications.