I recently purchased a 98 F-150 Lariat with 4.6. I also test drove a 99 XLT with 5.4 but the wheels would spin too easy in snow so I really didn't care for it much. Anyway, the 4.6 has 90k on it and is from the Romeo plant. While under load (being in drive) between the 1500 and 1700 rpm range there is a vibrating metal sort of buzz coming from the driver side near firewall area. If I just rev in neutral it's nice and smooth sounding. Is this a common engine related issue or is at as it sounds, something that's just loose vibrating? Thanks.

 

I had a similar problem with my superduty... turns out it was a cracked heat shield on the y-pipe. i just went under it with a pair of tin snips and some vice grips and finished what was already started... no more rattling.
I'd probabaly look for a loose or cracked shield. the cracks arent really easy to see... i found mine by tapping on the exhaust pipe with a mallet and listening for where the noise was comming from.

 

that's probably what it is. On my Duster the heat shield corroded around the bolts that held it in place so it fell ontop of the exhaust pipe. Been too cold and windy to do much investigation though.

 

Also chk the exhaust manifold stud/bolts and make sure the metal gasket is okay betweent manifold and head.

Rich

 

I have a similar noise, which is rather pronounced on cold mornings, under light throttle at 1700-2000 rpm.
What I believe is the same noise can also be heard while towing, hill climbing or under heavy acceleration. It seems to start at 3/4 thottle to full throttle, at any engine speed. In neutral there is no sound, and when the throttle pressure is released the noise stops immediately.

1998 F-150, purchased new. It's always made the noise when cold, but now it seems to be getting worse.
Taken to dealer who could not hear any noise, every 3000 miles until warranty expired. Nothing has broken (yet), but the noise is a big irritation.

Any ideas? I have been looking for any loose parts that might vibrate, but have found nothing.

 

mine does same thing.

 

I had a throttle body service done at 70000 miles, and now the noise is gone.