My '03 F250 6.8 has a stock exhaust (93k miles). When I'm towing above 2k rpm there is a howl (that's the closest sound I can match it to) that sounds like it's coming from under the hood. It increases with higher RPM. I don't think it's coming from the muffler, as when I have the windows down it still sounds like it's coming from the engine compartment. Is this normal? It's at least as loud as a diesel when I'm climbing grades while towing. Kinda irritating. I love the truck but wish it was quieter when above 2k RPM. I haven't been in any other 6.8 rigs, so I don't know if it's normal. Would headers help or make it worse? My last TV was a Jeep with a 4.6 V8 and all I could hear from it was a throaty exhaust sound. Your thoughts would be appreciated.

 

I'd check over your y-pipe exhaust connections, and the manifold studs to make sure there's no leaks. Mine has a few broken studs and doesnt have a CAT on it, and i find its louder than a diesel for sure between 1.5k - 3.5k rpm but quiets down past that

 

A lot of folks have had a pretty loud harmonic vibration from the torque converter sheetmetal cover on the front of the trans around 2300-2700 RPMs. Try either tighing it or removing it. It could also be your fan locking in more than it should, I know my fan is loud on the rare occasion that it locks in.

 

The inspection plate is more of a tin can rattle