Nodules on an old Hazel tree...what on earth are they?

The other day, I was removing root suckers from a very old Hazel (Corylus avellana) and I noticed these:

No, I have no idea what they are.

They appear to be hard, round, nodules: about the size of garden peas, and scattered all along the base of the tree, where the base flares out.

I've never seen anything like this before, and a cursory internet search ("nodules on hazel tree" "hazel bark galls") didn't throw up any images of anything similar.

So they don't appear to be galls (which I would normally expect on leaves, not on bark, anyway), and they're not burrs, either.

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