Make sure the plumbing is properly vented and be sure all lids are on correctly. If the grease trap is inside a room, make sure the room is ventilated. You can also increase the frequency of your grease trap cleanings. If possible, monitor what goes down the drain. You can try to chemically neutralize or mask the odors with bleach or other cleaner for a short time. Odor origins are very hard to pinpoint.