Got the car up on a lift (friend owns a lift) and found a chunk of substrate blocking the exhaust.
Front face of rear cat with chunks of precat littering it. Has sounded that way for ages.
Blockage.
Front face of rear cat.
Not shown: Melted down precat and chunk of wayward substrate liberated from exhaust.
Think this may have been part of my problem?
I'll check compression and sensors tomorrow, it was 105F today.