Quiet space heaters can be just as effective as their noisier counterparts, and the noise level of a space heater doesn't directly correlate with its efficiency. Quiet heaters may also have a psychological advantage, as users are more likely to use quieter appliances than loud ones.
Space heaters can be divided into two general types: radiant and convection space heaters. Radiant space heaters force an excess of electricity into a wire element, causing the wire to dissipate heat outwards. These heaters can heat up a small space quickly, but they don't retain much heat after being shut off, and they don't heat up large areas well. Radiant space heaters generate little noise.
Convection heaters rely on moving hot air away from their elements, sucking in cold air from the room and then in turn heating it up. Ceramic convection heaters use noisy fans to suck in the cold air and blow it over the heated elements, but oil-filled convection heaters use simple physics to draw in cold air over the large heated fins, heat it up and push it out.
Fans are the source of most noise in a space heater, but they do not necessarily determine the effectiveness of the heater.