I have a 2000 Ford Expedition 4.6L. Noticed a loud humming noise coming from rear near gas tank when I turn on the air. It keeps humming even after I've turned the air off until I actually shut the truck off. It's not the fuel pump. Was thinking it could be the rear A/C compressor but when i shut off the A/C it doesn't stop humming. What else could it be? Please help!!!

 

There is no rear A/C compressor, both the front and rear units share the one compressor under the hood. You're most likely hearing the rear blower motor. Blower motors never actually turn off even if the system is turned off. According to federal guidelines, they must run to keep the interior supplied with fresh air whenever the key is on. One other possibility is if you have heat/cool seats and you might be hearing the fan in one of them and it only sounds like it's coming from the rear.

 

I finally figured out the humming sound on the truck. I had a really bad original u joint from the drive shaft to the diff.
Also, the breather housr broke off thr rear axel making the diff fill with fluid causing my bearings to wear abnormally.
New joint ans a bearing kit the truck is like new. Just figured id update since it's been resolved. Thanks to everyone for the help.

 

Have you been driving this guy for the last year and half without repairs?