My usually calm rabbit has started thumping nonstop—what could be stressing him out?

I’ve had my rabbit, Miso, for about two years now, and he’s always been the chillest little guy. Lately, though, he’s been thumping his back legs almost non-stop, especially in the evenings. I haven’t changed his diet or routine, and I haven’t noticed any new noises or disruptions in the apartment. I tried giving him extra cuddle time and hiding spots, but nothing seems to calm him down. I’m worried because the thumping feels like a clear sign he’s stressed or scared, but I can’t figure out what’s triggering it. Could it be something subtle I’m missing, like a smell or vibration? Has anyone else dealt with a sudden shift in their pet rabbit’s behavior like this? How did you figure out the cause and help your bunny feel safe again?