Those scent machines make a small squeak noise when the nozzle is depressed, but by the time and deer get close enough to react to it, you could have shot them a dozen times or so. The sound is more noticeable on cold, calm days, but it is not very loud. I've had some bucks walk up to within 10 feet of the thing and not spook when it discharged. Does are nervous around it sometimes, though that may also be related to the estrous scent being sprayed.
All I know is that I have seen many more bucks at closer ranges for longer periods of time than I used to before I started using one.
Luck Counts, good or bad