I have a 98 Expidition and after driving it for a while, a humming starts from the rear left side. When the vehicle stops and is turned off, it continues for a short duration, then stops. short being about 10-30 seconds.

I have identified the source of the noise as a small 1' long cylinder about 6" in diameter on the rear left side in the undercarriage, but I do not know what this is, so I don't know where to look for troubleshooting it.

Tahnks for your help!

 

Could it be the air suspension? Try turning off the air suspension to see if the hum goes away. It's been a long time, but I think it's either a switch at the top of the front passenger seat footwell or it's inside the rear cargo hatch behind a panel on the left side. Sorry, my '98 Expy was 5 cars ago!

 

that seems to make sense, thanks, I will try that.