mmh, mmm, uhn, uh, ooh, aah, ugh, mmph, nggh, unh, whh, ooff, grrr, hnnng, ehh, yeee, aaauuugh, wooo, shhh (with a moan-like inflection), hmmm (prolonged and low), nyeah, yow, ooohhh (drawn out), glug (can be used to suggest a guttural moan), aahh (with a breathy quality). These can be combined and varied in pitch, intensity, and duration to create a wide range of moaning sounds. Consider adding subtle variations like adding a slight tremor or vibrato to the sound to make it more realistic. Experiment with different vowel sounds and consonant combinations to find sounds that fit your specific needs. Remember to consider the context and character when choosing the appropriate moan.