Humming when exhaling during sleep

My four year old has been coughing on and off today. We had Covid back in December. Now he is asleep and he is like humming in his sleep and moving around more than usual. He sounds congested too, but when I look, his nose isn’t running or anything.

I’m very sensitive to caffeine, and I thought decaf had none in it so I had 4 instant packs. My anxiety is through the roof and I’m terrified. It doesn’t help every time I go to sleep I have a nightmare within minutes that wakes me up. Like I dreamt my toes and fingers were rotting off. 😭 Now I’m worried something will happen to him, although I looked it up and it said it’s pretty normal to hum while he sleeps.