Tapping Inside Exhaust

[lute]

''96 Sierra C3500 7.4, 185k miles.
Happened to be standing next to passenger door when truck was started. Fairly loud tapping noise coming from under truck. Got under and found (much to my relief) the tapping was coming from a spot inside the exhaust pipe a few inches past the downstream oxygen sensor. With hand on pipe felt like a loose bolt rattling around in one spot.
Drove it home and rechecked; no more tapping. So I'm thinking that whatever broke off was staying in place at idle, but once the revs went up it got blown to a spot where it is no longer moving around.

The 02 sensor is the original, 20 years old, and rusted in place. Don't want to attempt removing it for no reason. Would it most likely be a piece of the sensor having broken off in the pipe?

[txab]

O2 sensor, piece of the cat, etc...

[Doug_Scott]

If it is past the catalytic convertor it maybe a chunk of that. It is more common for the honeycomb to break up and rattle in the convertor housing though. Once it starts breaking up you will likely experience occasional loss of acceleration due to the broken up honeycomb rattling around to a position that impedes exhaust flow enough to block the exhaust. Saw that a few times over the years, and had it happen on my 87 5.0 Mustang. Got lucky on the Mustang, it was just short of 5 years old so it was a free fix from Ford.