Wait a minute, you said you put Pereseus as wingman loadout. I remember you don't have Perseus as loadout...
My guess is because the Maras (Terran) are noted as Terran in the ship.tbl. The sound.tbl is set that every Terran ship has Terran engine sound. Therefore, the Maras engine sound is supposed to be Terran. But then the makers changed the sound of each ship to be Shivan. So then you get shivan engine sound now. However, this applies for the whole loadout, meaning whichever ship you chose would have the shivan sound.
This is a theory. I personally don't know if this is true, I don't even know if this could be done, because I'm am currently 385km away from FS2. It may be a bug, or it may be not.