Today I had the alignment done and while it was on the 4 post lift I had a look underneath to take a picture. It turns out the arb is only touching the diff mount when the suspension is on full droop. When it's on its wheels there is about 1 - 2 mm clearance. Unfortunately the picture I took was blurry because it was too dark.

So I'm not sure where the noise is coming from, but I also didn't hear it today... I will keep an eye on it whenever I drive it again.