Remeber these days are often blue sky, so you get morning heating on the surface, causing thermal turbulence in air, which builds up and rises, but when insufficient to break through a strong inversion, it gives a layer of disturbed air just under the inversion top.
This can be quite worrying to get vibrations after a long perfect flight above the inversion just after you start the descent. If you're not expecting it, you relate it to the thing you last 'touched' - and that was the throttle reduction to start the descent - conclusion - something wrong with engine!
(Of course you have to remember how S### Law out-trumps anything - now you know this, on the day you knowledgeably put the vibration down to inversion turbulence, it will be the day you don't have carb heat selected before descent, and it will be the engine)