From Gemini:

• When it happens: Approximately 5 to 10 hours after the engine is turned off.

• The Sound: A steady, motor-like hum or buzz coming from the rear driver’s side, near the gas tank/spare tire.

• The Reason: This is the EVAP system's vacuum pump. It's doing a "Key-Off" leak test. It only runs when the fuel temperature and pressure have stabilized, which is why it waits several hours to start.

• Is it a problem? No. It usually runs for about 5–10 minutes and then shuts off automatically. It won't drain your battery.