It is very difficult for other forum members to provide feedback without hearing the sound. Joel, for example, shared in his reply an audio track which he called clicking, there is without doubt some sound there, but it is very confusing to use the word “clicking” to describe it, as clicking (at least in my mind) is normally a sharp metallic sound. It was so helpful to hear it as opposed to reading a description!
It is particularly intriguing that you write that you hear a knocking sound while coasting, i.e., even when you do not rotate the pedals. It must then either come from the spinning flywheel, or from a shaking frame of the trainer.
Your best bet is to open a ticket with Wahoo support and upload an audio (or video with audio) and ask for their advice. My experience with Wahoo has been that they reply quickly, give good advice, and overall provide great customer support. One of the things which they might advise you to start with, is to check that all legs on which your trainer stands are in tight contact with the floor and the trainer does not shake from leg to leg. The legs can be adjusted by rotating them.
If Wahoo decides that the trainer requires a repair, they probably will ship you a new one. When I had an issue with Wahoo Headwind fan (chemical smell from the plastic), they cross-shipped me a new one right away.