Intermitten buzzing sound on HWY

This might belong in "modified" tech but I don't think so.
The other day I was driving the wife & I to the midwife.
Around 60 mph I notice a buzz, for all the world it sounds like the fan plates are making contact with something, that came & went a few times over the 2 miles of freeway driving on our trip. It never happened on the lower speed part of the drive.
This morning I look under the hood and see plenty of space around the fan & no witness marks on the shroud or radiator. So I don't think it's actually the fan...
What else can we think of that makes this type of noise?
I've done too much recent work to list it all and it doesn't matter anyway... early parts failures do happen.
Again, I'd have to describe it as sounding like fan blades hitting something, a real fast buzzzz. I didn't feel any vibration that I recall, just a noise. Almost like hitting a rumble strip but higher pitched? I'll see if I can describe this better after my drive to / from work.

Just throwing this out there, mine is making a buzzing/very fast metallic rattling noise under different levels of load. It's the loose exhaust hanger to the left of the oil pan and bellhousing if looking at the the front of the vehicle.

I found this by while the engine was running (in park with p-brake on) pushing up on the hanger with a screw driver - noise went away. Only reason I offer this up is because mine does not do it all the time, it varies based on speed and terrain. I plan to tighten mine up shortly. Worth a look.

Another idea - I also had buzzing under acceleration at any speed and it was my upper fan shroud that was beginning to "lean" to close to the top of the mech. fan. I screwed a small hole in the top corner of it and zip tied that sucker back to the hood latch - problem solved. This also would only show itself while accelerating because the engine was moving on the motor mounts causing it to contact the shroud.

my 90 XJ started "buzzing" on my last trip to Moab, it took a couple hundred miles to figure out.... it was the chrome trim around the windshield vibrating. a couple spots of RTV squished under the trim got rid of it.

How's your motor mounts?

OEM motor & tranny mounts are cheap & easy. After market mounts (Brown Dog) are quite a bit more expensive but I'm told they're bullet proof and will last forever. No personal experience with them though.

If the engine is moving side to side it's motor mounts...if it's moving back & forth (see sawing) it could be motor mounts or more likely tranny mount (or all of the above) allowing the rest of the drive train and the fan to lean back and contact the shroud and/or radiator.

My advice FWIW, do both, fix it before it eats your radiator.

I'm running OEM style motor mounts less than a year old and can't find any witness marks around the fan / shroud / radiator.
I also only got a hint of buzz yesterday AM and none on the highway last night.
I'll be looking around the Jeep some more for anything loose or that looks like it's made recent contact though I get the feeling this might take a while to find - anything I can't duplicate pretty much on demand seems to be a PITA.

Paradise, whats the best way to check for motor movement? I recently started a thread about the "Shuffling" sound I was experiencing. Mine turned out to be the mechanical fan contacting the shroud.
Do you use a pry bar to gently check for movement?