I get a noise for the engine compartment when the truck is off. It sounds like a inexpensive air compressor. Also I get the same noise when I start the engine sometimes. It is a 98 expediton with a 5.4. Truck seems to run fine otherwise.

 

Do you have a cold air intake? Mine makes that noise when I turn it off after doing this upgrade.

 

No, I haven't changed anything on the truck

 

Does it have air ride?

 

Does it make the noise no matter if the AC was run or not? Note AC may run in any position except Off, Panel, or Floor. A fully pressurized AC system & hot engine can turn the compressor over for a little while "when truck is off".

 

The sound when you turn off, is it like a humming , is something to do with the alternator, and is normal.

When you start up, could you be hearing the fuel pump starting up, it sounds like a humming noise also and normal to.

 

It might be the air suspension compressor?