What the heck is that running after the ignition is off?

My teenage daughter’s 2002 GMC Envoy has some kind of electric motor running under the car, even after the ignition is off and the key is removed. The only way to stop it is to disconnect the battery. It’s not the cooling fan or the heater blower, it seems like its in the transmission. It could be a electric fuel pump on the rail, but its much louder.

Ideas anyone?

My best guess is that this is something involved with the Evaporative Emissions System, as many of them will run a test of the mechanism after the car has sat idle for a few hours. However, you didn’t tell us if this situation is occurring immediately, every time that she shuts off the ignition, or if it seems to occur after a few hours of sitting.

Does her Envoy have an air ride suspension? The suspension air pump can run unexpectedly if there is a leak in the system. It’s usually located on the front of the vehicle just behind the bumper.

The TrailVoy platform has an A.I.R. Pump mounted on the inside of the driver’s side frame rail directly underneath the driver’s seat. The computer commands how hard it runs and at full power can be quite loud. Check to see if it’s running all the time with the engine running too. If it is, check the relay associated with the A.I.R. Pump. No matter what, it should stop after Key Off for 5 minutes when the Retained Accessory Power (RAP) times out. Might be interesting tidbit to know if it does or not if the relay checks out OK.

My first guess would have been a tank-based fuel pump. Some cars the fuel pump will run to re-pressurize the fuel rail even if the engine is not running. If it is the fuel pump, and not noticed before, there may be some kind of problem with the fuel pressurization system. A bad fuel pressure regulator, etc.

My second guess would be as mentioned above, the evap system. It keeps the pressure in the gas tank at spec’d values, either a slight over-pressure, or a slight vacuum, varies car to car. So if this is the source there is a motorized pump which does this function. Again, if this seems like a new noise, and the source of the noise is the evap pump, there may be a problem in the evap system, some kind of leak or a bad purge valve maybe.

Other comments above are a possibility too.

Does this vehicle have air suspension. It could be the compressor running on. There are 2 locations according to the wheel base.

Under hood fuse block;
Fuse #01 ECAS (Electrically-Controlled Air Suspension)
Fuse #26 Engine 01 (EVAP Cannister Purge and Cannister Solenoids
Pull these fuses. (One at a time) If the sound stops, you’ve found the problem.
The ECAS air pump is in the rear passengers side above the tire.