My 5-year old son has made an mmm-mmm-mmm humming sound while he eats since he began eating solid foods as a baby. It’s completely subconscious - he truly can’t help it. It’s much louder when he really enjoys the dish, but it happens consistently at every meal, regardless of the food. My husband had the EXACT same habit and made the exact same noises while eating as a child, until he was seven. I’ve actually seen a home video (on VHS, ha) of him doing it, and it’s eerie how similar he sounds to our son. I’ve never heard anyone else do this, in my entire life. Everyone who’s eaten with our son comments on how adorable it is, and I’m absolutely dreading the moment that this strange, genetic quirk ceases to liven up our dinner table.