Bizarre Rattling Noise

[MechDuck]

Hello all, this is my first time posting on these boards. Too bad I only found them after I went looking for answers. I have already checked on the board, and even if I have seen the same issue, I may have overlooked it if it didn't seem like the same thing. Sorry in advance if I've missed something.


I have Googled this up and down, and can't seem to figure it out. Figured maybe y'all could help me out.

The details:

1999 Yukon SLT
4x4
~126,000 miles


The Issue:

Sort of heavy metallic rattling/ringing noise coming from what sounds like (I realize sound direction is often misleading) under the passenger front floorboard.

What I have found/tried:
ONLY does it after the vehicle has been driven a distance.
ONLY does it during acceleration.
-When idling, or cruising at speed, it does not do it.
No detectable vibration or change in performance associated with it.
Does not matter if I'm running RWD or AWD, I get it both ways (again, only when the vehicle has been operated for a distance)
Transmission fluid is clean, and at normal range.
-No burnt/hot smell at all.
Engine oil is at normal range, changed recently.
I have been all up and down the underside of this with a rubber mallet checking for anything loose that might rattle, to no avail.


If anyone has any ideas, I'm all ears. I do most of my own maintenance, but I'm no pro mechanic by any means, so I've probably overlooked something.

Thanks for any help!

 

[OR VietVet]

I would check for an exhaust leak. May be from a broken exhaust manifold bolt or 2. Exhaust shield in that area. Maybe from cat shield.

 

[ccapehartusarmyINF.(ret)]

im probably way off but im just going by yer description of a mettalic noise so id
check your front wheel bearings i just replaced a bad wheel bearing and before it went kaputt it was making alotta metallic rattling type noises and it sounded like it was coming from the same area and it made the noises at specific times like you described
just jack up yer front wheels and try to shake them if there is any play if there is the bearing is bad
also i noticed on mine the noise sounded like it was coming from the front right area there but it was the wheel bearing on the opposite side so the sound can be misleading

 

[2000tahoeZ71]

Im with raider, its likely a cat shield or the substrate is loose.