I have little furry toys shaped liked animals with cat nip in them and my cat carries them to my bed, sofa and all around making a loud meow. To me it is her I caught something and here it is as a present sound. Maybe yours is muffled because of the blanket.

 

Sounds like he's blowing off some steam, releasing some tension...

Or he could be showing off. When Robin was a kitten he'd pick up a big sock and carry it, prancing around. 'See how big and strong I am?'