How noisy are they?

The oil balanced flues are very quiet, just a rumble in the background.

The flued models are always noisier as you can hear the noise through the steel flue pipe and it resonates a bit when inside a fireplace, so it depends on your set up. The noise is only there when the burner is firing, which is by no means continuously. Most people find it a comfortable low rumble, there is certainly no high pitched whine.