Bwahaha, it means he wants to mate.. or something like that

Or he is INCREDIBLY HAPPY to see you.. it must be time for him to get neutered. How old is your Zeus??
My little ChaCha used to make that noise ALL THE TIME around me, it sounded like 'mm, mm, mm." It was the MOST adorable noise ever!!
She got spayed in May, and I still hear it, it isn't quite as often.