In operation, the fan is not noticeable and if it needs to cool down a little more, then a very slight hum is audible, but only when watching TV very quietly.
What bothers me is the high-pitched beeping when it's switched off. We have the projector in the bedroom, so of course it's noticeable when it's otherwise absolutely quiet, but if you have the projector in the living room, this beeping is no longer noticeable.

I've had mine for 6 months. As long as the filter has no dust in it, it is super quiet. I had to vacuum it from the outside after about 4 months, but since then it's been whisper quiet again. I cannot yet judge how the device behaves in the hot summer.

Depending on the location, this can be quite disturbing. I have now built a lowboard for the beamer. To allow the exhaust air to escape, I drilled openings on both sides (furniture). Since then, the fan can only be warned at a low volume.

I am very happy with my white Optoma beamer and don't find the ventilation annoying at all. To be honest, I didn't even consciously notice it.