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My 2004 Chevy 1500 is making a humming/motor noise after standing in the rain

What could be causing this?

I’ve had the truck for ~almost~ a year, and it’s never done this before (and we’ve gotten plenty of rain, btw). The humming sounds like a small [electric] motor running, and it seems to be coming from under the driver’s seat area. We noticed it this morning, and the truck started and backed just fine. It continued THROUGH the starting/backing, and is still going, though the truck’s turned off.

Of course this is happening on a Sunday… Help?

Could it not be car related at all? Like a childs battery powered toy under the seat? Narrow this down a little, do a visual inspection and give us more to work on.

Do you have electric seats? Do you think you are directing us correctly when you are relating the noise to the drivers seat area?

To address your first idea: no, it’s not a toy. I do have electric seats (I seem to have electric everything), and it’s not something in my door hitting the seat control and making it hum, as the noise continues with the door open.
It stopped for a while today, and it’s been raining/sprinkling/pouring all day. We ducked out to investigate during a brief let-up, and came to the conclusion that it is NOT in the cab area; the sound seems to be coming from under the cab area and not from under the engine or bed. Then it started lightning and pouring again, and here I am.
Sometime around noon, the sound stopped. Sometime between noon and now [four] it started again. It is not a constant sound - it “revvs” up and down slightly.
According to my [somewhat car-savvy, in the older models] husband, it may be the fuel pump… but he is confounded, since "nothing should be running with the ignition off."
Any ideas??
Thanks, by the way.

When the noise is present remove the fuel cap a listen into the tank,can the noise be pinpointed into the tank.

Something else I can think of but I am not sure one is used on your truck,that is a “leak detection pump” it pressurizes the fuel tank and checks for minute (or big)leaks. Why it would be running with the key off is unclear.

Having what sounds like a electrical motor run when the key is off is not a good sign,possibly a sign of a electrical short,not a short to ground but a short to another system.

This may or may not be related to the situation with your car, but it is food for thought:

On recent model Toyotas, the evaporative emissions system runs a “self-test” under certain conditions, i.e.–after the engine has been shut off for at least 60 minutes, if I recall correctly. This self test involves the fairly loud running of an electric pump that is located on the underside of the car, directly beneath the driver’s seat, and for a few minutes, it results in a noise very similar to what you described.

Now, of course, a Chevy is not a Toyota, but it is very possible that this vehicle is also designed to run a self-test of the evaporative emissions system under “ignition off” conditions, and if that is the case, it is possible that the self-test is not terminating when it should, hence the ongoing noise situation. This could also indicate a problem with the vehicle’s evaporative emissions system that is causing the self-test to run overtime.

How did I find out this information about Toyotas?
By reading the Owner’s Manual of my friend’s '08 RAV-4!

Have you read your Owner’s Manual? If not, this is just one indication that you need to do so. With any luck, it will give information similar to what I read in the Toyota manual. If it turns out that your truck has a similar self-test, at least this will give your mechanic something to zero in on.

You should also read the details of the Emissions Warranty on your truck, since it is possible that this part is covered for an extended period of time. Check this detail before you go to the mechanic, because if it is covered, then you need to go to a GM dealership for a free repair.

Read that manual as well as the details of the Emissions Warranty, and report back to us!

P.S.–Is the Check Engine Light (CEL) lit up?

Another forum’s user suggested we check the ABS fuse, as a problem with ABS relay system seems to be common with the 1500. After pulling the fuse, the motor hum sound died off, and stayed off. Of course, we don’t have ABS brakes now, but the brakes are working… so I’ll limp to the shop and get the EBCM checked.

After that brake-checking test drive, the CEL has been confirmed as to being unlit; thankfully, it doesn’t seem to be an emissions-control problem.

And, I’ve read parts of my owner’s manual… Chevy seems to be set up for on-going diagnostic for emissions while running, and I didn’t find anything pertaining to an emissions test run after the ignition is turned off. I’ll keep studying-up on this fancy new truck. Thanks.