On my '07 Expedition EL 4wd with 212k miles. I have a rattle that I can't pinpoint.
I don't think it's detonation but I could be wrong.
I tried using 95 and 93 octane and it made no difference.
I've slid under the truck a few times, I used a rubber mallet and tried to manipulate the sound or find ANYTHING loose but everything including the heat shields seem tact and/or screwed in tight.
On top of the skid plates I found and removed about 10 lbs of stones, I thought for sure that was going to be the solution...but it wasn't. I was surprised because the stones sliding and vibrating around had pretty much the same sound.
The only other thing I can think of is the cat is falling apart internally. The truck is in mint condition especially for the mileage, the undercarriage is so clean and rust free inlcuding the exhaust... but I can't know what's inside the cat

Some details that may help with the guesswork.
It ONLY happens when all 4 conditions line up:

Only happens on acceleration. Between 70-80% load to be more precise.
Only between small window of 1800-2200 rpm's
Only between 20-40 mph.
Only in 2nd and 3rd gear.

It doesn't matter if its cold or hot out, or if the engine and exhaust are hot or cold.

If I let off the throttle when it starts to make the sound it quickly fades to no rattle at all.

It's more quiet and longer lasting at 20mph and it's slightly louder and not as long lasting at 40mph

Thanks!

 

I don't have an answer for your problem, but holy heck, 212k miles???? I've had my 2007 EL 4X4 as the original owner for 7 years now, and at my current rate of driving, it would take me another 13 years of ownership to make that mileage (year 2027). I'm sure many on this forum are curious what you've done for maintenance to make it this high mileage, and what repairs you've had to do so far. I plan on this being my last full size SUV that I'll own, so I want to keep it for a good many years to come. Good luck with your diagnosis.