The only thing I've done under the hood currently was upgrade my alternator. When I rev the engine, everything is smooth, the sound kind of drowns out but once the engine gets back to normal idle, its still there. I shook a bunch of things thinking something was loose, and nothing. I thought maybe something was hitting something, no. Does anyone have a thought what this is?

 

How about a clue as to what the noise sounds like?

 

I just went back out to listen again. it seems like it dies down after a minute or 2, but didn't earlier. the noise is like a generator running in the background, and right when I switch the view tot he drive belt you can hear it lighten, and right at the end the cam view goes back up slightly and it picks back up.

I didn't hear this the whole week after the alternator was installed, so this is new.

 

Your best bet is to remove the drive belt and then start the engine to see if the noise is still there or not. This will help you to confirm or eliminate the engine as the source of the noise. If the noise is gone with the belt off, then with the engine off you'll need to rotate each pulley the belt rides on to see if any are stiff, turn rough, or turn hard.

 

im going to due that today. for some reason I didn't do it while I had the belt off earlier.i know its not the alternator or the water pump. a stethoscope was put to it and it sounds like its on the top left rear of the engine. the sound couldn't be located. Im also going to shake things under the truck to make sure its not a echoed rattle.

 

Maybe I can help with this problem. Have a 5.4 Navigator 98. L series engine. Had a strange a bit pinching high freq noice from under the hood, passanger side. But never on the first start of the engine.. Always on warm start. Turnes out that timing was bad and needed to be replaced... Lot of effort and money but away...

 

How do you replace "timing"?

 

Well... I dropped my car to the mechanic and pick it up 2 or 3 days later

Not sure if the word was acurate - what I meant was that I replaced the chain, gears, slides and all that stuff...