I noticed a weird noise only on acceleration. Like a rattling when revving high. Also my performance has been bad for a while now, it seems like the throttle wasn't very responsive at all when reallying pressing the pedal down...way too much delay + a rattle sound that I thought might be the driveshaft. I thought to try seafoam.

After running seafoam through my brake booster trying to get the truck to run better, I spotted the exhaust leak as the carbon deposits were burned where the flange gasket sits between the headers and the exhaust.

After removing the old flange gaskets I compared them to the new ones. My 99 Hoe has 144k miles. I wouldn't doubt that this was the original flange gaskets I removed. They weren't as thick as the Duralast ones I got from autozone.

Replaced both flange gaskets and she runs smooth as butter now. Seafoam is also a great way to find exhaust leaks.