Exhaust rusting.
2004 TL wagon: Rust holes appeared at both the resonator end plates. Replaced by exhaust shop with new aftermarket part, $210 charged.
2001 TJ sedan: same middle section, from cat to rear muffler. Thought I would do this one myself. This time there was a big rust hole just before the joint to the rear muffler, but most of the noise was coming from the front resonator. Front plate had let go of the pipe, plus small rust hole in front plate at bottom. Replaced with Redback part, $179.
Not a difficult DIY job, used spray to loosen rust off bolts and nuts, also used on rubber hangers to make them slip off and back on.

Both replacement sections have hotdog style resonators, ie no separate end plates, so may last longer. No heat shields or earthing straps on them though.