Cat converter is the most likely source of the described noise.
Listen for the noise when engine is cold, it takes a few minutes for Cat to heat up, so if no noise then it could be Cat.
Noise after shutting down is the cat cooling off.
Could have a broken seam or ??
Popping noise is the thin metal expanding and contracting, and yes it could get worse as outside temp went up.
Cats run hotter if your engine is running Rich, hows your MPG?
If it has gone down lately then Cat popping could be the sign of another issue.
Maybe O2 sensor is starting to fail lean so computer adds more fuel than needed, recommend replacement for front O2s is 100k miles
The other culprit may be torsion bar bushing(s), in 1998 some 2WD rangers got torsion bars instead of coil springs up front.
These work by twisting to support the weight of the vehicle, but usually this type of issue would be noticed more on uneven surfaces, bumps, and a clunking noise not popping noise
Last edited by RonD; 05-19-2015 at 10:30 AM.