Older ferret grumbling during sleep

I've got an older (7ish) male ferret who has recently developed an odd quirk; he's been making distressed sounds during his sleep. If it was just once or twice I'd think he's just having nightmares, but it's been pretty consistant for about a week. He does it a couple of times an hour until he hits deep sleep, then he stops.

He's not off his food, or his water, his poop is normal, he's sleeping about as much as he always does (he's quite lazy for a ferret), no solid lumps in his tummy, no sore spots, gave him some chicken paste to move any hairballs, his gums aren't pale, and he doesn't make these sounds whilst he's awake. He was coughing whilst awake a little the week before infrequently, so I'm assuming he's caught something sniffly and small.

If it doesn't pass soon it's a trip to the vet of course, just asking around to see if anyone knows anything. I've not had a ferret be this vocal in their sleep before, and its very odd.