right. It is when the spot welds break (after many many miles of driving) that the insert starts to rattle and produces that wonderful noise.
I doubt there could be any "shrinkage" going on in there, cause they will expand while the car is warmed up, then contract when the car cools. I don't see how the diameter could shrink over time.
I'm pretty sure the tips are packed with fibreglass as well, maybe that stuff erodes away over time and gives the insert room to rattle around.
Find the welds and crack them? ;)
Or wait for nature to take it's course.