If it is a rat tailed maggot,and it sounds like it, it is of the genus Eristalis, which after pupation becomes a large hover fly of the bee mimic type.
They live as larvae in rich organic material including rotting corpses
and some think they are the origin of the Biblical story of "From the strong came forth sweetness".
The idea being that the flies from the corpse of a lion were mistaken for bees, which, of course make honey!