Continental Giant thumping

Hi folks. Any suggestions please?
My 5 year old Continental Giant has started thumping recently. The first time went on for about an hour, every 5 to 10 mins, there was a single thump and he made a small noise, like a hurumph as he did so. He was safe indoors at the time and his friend was unaffected. This has happened twice since, in the dark. Then yesterday, he was outside, in the day and he thumped for approx 10 mins. Each time I checked for dangers, but saw nothing.
I am wondering if it is a pain thing? He is on Metacam and Zantac, for his joints. He generally eats and poos well, but does go down a bit after a thumping session. Thank you.

Was there definitely no dangers, or noises that could be perceived as danger around?

My bridge bun Rio used to thump.Him and his wife were total house rabbits.They used to lie under a table beside the living room window.There was usually no obvious cause.I assume he heard foxes/cats etc.Guy Fawkes night was noisy but they would sleep through it.

There are definitely no predators nearby. There might be in the field next door, but that isn’t too close. I was wondering if he might be losing his eyesight and thumping, due to that?