Be aware that the lowmmmmmmmm sound might be the beginnings of a yodel. This is how our boy started. He didn't yodel for the longest time, and now he only does it when someone comes in. The longer it has been since he saw you and the more he likes you, the more vocal he is, but it all started with what we thought was a growl. Then we noticed his body language, ears slicked back (whippet ears) and tail wildly wagging, running in circles, etc
Does he like your friend? If so, this might be what he's doing, and you don't want to discourage him from the beginnings of his yodel!!