Groan at the graveyard explained
The groans I heard at the mauseleum was gas escaping from a decaying body. Well meaning loved ones sometimes put their loved on in a sealed coffin with the hopes that it will keep the body from decaying. That is a mistake. The bacteria present in the corpse will decay even an embalmed body even in a sealed casket. On rare occassions coffins will explode because of the pent up gas.
If the body is ever taken out of the coffin it will look worse than if it were a skeleton.