You don't take this as a sign of appreciation?
It is. And he is definitely enjoying the food. But when its literally every few seconds its enough to drive me mad!
Yep, my MIL started doing that in the early stages, and progressed to humming tunelessly to herself.
He hums too, much of the time. Not so much tunelessly as repetitive humming phrases. Nothing I can do about it other than engaging him in conversation. I think the noises are a type of OCD and serve to reassure him.