Loose heatshields would be my guess. Especially if it's a factory cat. Those babies start to rattle in time.
You can check and verify the exhaust like the other user suggested as well as verify your exhaust pipe isnt touching a crossmember as well. I had my exhaust pipe occasionally touch the crossmember after all the cats. Boy was that rattle annoyong..
might as well verify all your exhaust clamps are good too.
Sent from my SM-J337V using Tapatalk
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7 3rd gens listed in the build thread (2 are parts mobiles)
Build Thread:
https://www.toyota-4runner.org/3rd-g...os-builds.html
Brillo's Bucket Fluid Ex changer:
https://www.toyota-4runner.org/3rd-g...ml#post3358086
Sparks Plugs Wire and Coil Information:
https://www.toyota-4runner.org/3rd-g...on-5vz-fe.html