Teamspeak knocking noises?

My colleagues and I have been experiencing a phenomenon in Teamspeak for some time:

Two out of three people present hear a dull knocking sound at irregular intervals at the same time. It's a rhythmic, dull sound, similar to someone banging on a sofa or an insulated wall.

The noise is not caused by the third person's microphone, as it occurs even when muted.

We can't figure out where the noise is coming from. Perhaps someone here has an idea that's more technically explicable than a ghost.