While I was doing the replacement of my oxygen sensor (http://www.brickboard.com/RWD/volvo/1294600/740/760/780/1991_745_regina_oxygen_sensor_replaced.html)
I also decided to check into an annoying high-pitched rattle that has been bugging me ever since I bought the 745. I figured it was a loose heatshield or clamp.

After checking the usual suspects, I found nothing loose. The source of the rattle seemed to be the catalytic converter. I decided that the honeycomb might be breaking up and that I best check it before it got all plugged up.

After unbolting the flange (this was fairly easy - downpipe is a recent replacement) I found two things. One, the reason my exhaust is loud is that there is a large chunk of rot in the axle pipe (this led to my rear muffler falling off a few days later)and two, there is nothing wrong with the honeycomb.

The source of the rattle was an M10 bolt inside the converter on the engine side.

I've heard of "mechanics revenge" like this, but I always figured it was an urban legend. Now I know it isn't.

There is no way a bolt this size can pass through the engine without destroying it. There is no organic way for it to get there, it requires human intervention. Some people...

On the upside, now my car does not rattle and I hear less backfiring on deceleration.