For as long as I can remember (50,000 miles or so) my truck has been making the strangest sound and I cannot pin-point the source. I have been waiting for whatever it is to break, bu that has not happened yet and the truck runs great.
It only happens around 25-35 mph and real light throttle. Sounds almost like a mig welder or a wire shorting out is the best I can describe. I was thinking maybe tensioner pully but it sounds really close to the fire wall. Possibly just another one of those ford noises that cannot be explained. I don't want to start throwing parts at it so just wondering if anyone else with the 4.6 has experinced a similar issue. Any help is appreciated.

 

Is it almost like a whining sound while you are accelerating? In my 04 F150, (which also has the 4.6 V8 like yours) whenever I am touching the accelerator over 20mph there is a strange whining sound that seems to come from the front right part of the truck at the firewall. Similar to yours?

 

It is not a whining sound, but sounds just like a mig welder (that crackling or wire shorting shound) and when cruising not accelerating, around 25-35mph. I am thinking possibly the motor on the throttle body, I am sure it is not a plug grounding out because the truck does not run rough.
I will be looking for a local shop with a dyno to get a better listen under the hood. Or maybe get one of my crazy friends under the hood while I drive down the road

 

idler pulley is only $15... that took care of my whine

 

 

If it's a 5.4 3v it doesn't have an EGR valve.

 

Thank you, I am going to check this out and hopefully it solves my problem. It would make sense being so close to the firewall and that is the rmp range it happens. Do you know if there is a way to test it before spending the 100 bucks on a new one?

 

The OP said it was a 4.6

 

I'm not sure I'd recommend my test procedure, but, I will describe what I did.

My neighbor, armed with a screwdriver stethoscope, perched on the engine compartment. I put the engine in gear, with my foot solidly on the break, and carefully pressed the accelerator until the RPMs got into the "rattle zone". Using his long screwdriver, my neighbor listened to the EGR (driver's side near firewall) and determined it as the source of the noise. Apparently there was no question.

We could have easily become Darwin award winners (especially him)- but it was done in a safe area with complete trust between us.

 

Any update?