If you mostly care about sound, there are some that don’t have a fan.

One option is a fanless convection heater, like the oil heater Hikermick recommended. These things just use passive convention to transfer heat, so they don’t have moving parts. You might hear a few sounds from the metal as the thing heats up and cools down, or maybe the relay clicking on and off occasionally to regulate the temperature, but not aside from that.

They take a long time to come up to temperature, so they aren’t ideal if you regularly go in and out of the room, but they work fine for keeping a place at a more-or-less constant temperature.

Another is a radiant heater. These are basically big infrared spotlights, heat lamps. At least some can make a bit of sound when heating up – I had one that did this, and another that was fully silent.

These are good at heating up a particular location, whatever they’re aimed at, quickly, so they’re good if you just want to heat something in the room. You can aim it at yourself, have the heat immediately affect you; I’ve seen them recommended for things like bathrooms for this reason.

Unlike oil heaters, these will emit some visible light, so they aren’t ideal if you want the room to be fully dark (e.g. I don’t know if you watch movies in your office or something, but it’ll light the room at least somewhat).