What they actually found in this MRI study is that the bubbles stick around for several minutes, slowly dissolving back into the fluid. They don't immediately

What they actually found in this MRI study is that the bubbles stick around for several minutes, slowly dissolving back into the fluid. They don't immediately collapse as you would expect. The sound seems to come from the very rapid expansion. The time it takes for the bubbles to dissolve again seems to be responsible for the "cool-down" period that you experience between cracks.