[joeydurango]

Good question. Simple enough - I just have a mark on the driveshaft, and set it in a given position by hand.

When I don't hear the noise while driving, I look and see the mark in the same position afterwards. If I hear the noise, however, the mark has moved.

Worth noting that even if I stop immediately after hearing the noise, I can still turn the front shaft by hand - so it obviously isn't really locked in. That's where the "intermittent" and "floating" description comes from. It just comes and goes.