SR22 NA- About 3 months ago I started to feel a slight intermittent vibration when at cruise. I could time it and it would last 10 seconds and disappear. Over time this has grown to be 20-30 seconds at a time. Frequency has also been intermittent but the last two flights it has been consistent meaning vibrates for 20-30 seconds and then stops for 30 seconds. These are 2+ hour flights and it remained very consistent throughout even during descent.
We have checked cylinders, mags and spark plugs.
Any advise would be appreciated.