Hyper:
If lucky, the oil check valve on right side of turbine is slow to react to your power changes. If unlucky, a turbine labyrinth seal is moving during power (air pressure) changes as AC mentioned above. It really shows after landing when you roll the throttle to idle and you make enough smoke to screen the entire Royal Navy. On windy days a pilot wouldn't even know until the fire brigade shows because a witness thought you were on fire. Check valve is easy (cheap) troubleshoot, but mostly lacks results since a "new" style check valve came out 20+ years ago. YMMV but most would hold off on a turbine change until it became a mosquito chaser on the ground. There are a couple other reasons but these are the top two on my list.
W1