2005 F150 Lariat, 5.4 205K miles.

Original engine and transmission, no rebuilds or repairs on engine or transmission, just regular maintenance. Lately I've been having this "rattling" sound when under load when accelerating or going up an hill, when I get above 2,500 RPM's or so I don't hear it. Also when putting it in neutral when rolling down the road it's also quiet, I checked the heat shields on the Cats and also disconnected the vacuum hoses to the hubs to see if it was maybe the IWE's not engaging or disengaging. So far I have not been able to identify the problem. It does have the cold start rattle that goes away after 10 - 15 seconds or so.

 

It could be the cats themselves. If the internal honeycomb inside cracks and breaks it will rattle. Crawl under the truck and bang on the cats and listen if you can hear anything rattling.

 

This is a classic description of spark knock. Try a tank of Premium and see if it stops. If it does, go back to regular but with an ounce of 'Startron' in every fill-up. That means every single time you get at least 10 gallons, put it in there - from now on. The problem could be from a lean mixture from dirty injectors combined with a carbon build up.
Obviously there can be mechanical causes but they would most likely throw codes, like a failed knock sensor or ignition advance program. Going to a higher octane fuel and having it stop at least shows you that it is spark knock.

 

premium gas should take care of it, fuel is not like it was back in the early 2000's. my excursion does the same thing.

 

I call BS on that. Octane numbers haven't changed.
I believe the issue will be solved with a few cans of a modern fuel additive to clean the combustion chambers. I have a love/hate thing going with Scotty Kilmer, You Tube poster, but his recent video on fuel additives was instructive because he used a borescope and showed us results inside an actual engine. His point was tht the old standbys like Seafoam were made for different days, and the newer products like the latest version of Gumout will actually clean up carbon deposits.

Merry Christmas!
R.