I’m half way through a road trip and I’m starting to get a sort of rhythmic humming sound coming from the TD5. Before when I’ve had a wheel bearing go it’s always been a sort of constant noise but this one goes on and off. Could it be a single roller in the wheel bearing that’s failing? What are the consequences of leaving it? I’ve got about 20 hours more of driving, and about 10 until we meet up with friends (which would make changing it more palatable).

Is it a wheel bearing or could it be something else? We’ve just got in and it’s 2am so I’ll lift the wheel tomorrow but if I need to get this sorted now (hopefully not) I’ll need to get the parts on order asap.

If the rhythm varies as you turn the steering wheel some, it could well be a bearing on the way out.
Bearings can go on some time being noisy. I have just done one on my car as it was getting well noisy.
Have a feel of the hub flange after a drive, one could find one warmer than others indicating an issue.
A better check would be to jack up the wheels in turn and give a spin. Smooth good to go. Should you find one that has a wobble when pulling/pushing at 12-6 9-3 o clock positions then replacement is needed. Or you may able to tighten and lube it if you have the tools [ Td5 not the same as previous models as they have stake nut method ]
A UJ starting to go dry can make similar noise, heat check works here to.
Good luck, Take it easy.