Could be almost anything...
A while back, we were descending in a 747-2 when the yoke started a high frequency vibration.
It dampened out as we slowed and mentioned it to the mechanic on the ground.
Post flight inspection revealed an open panel on the #3 eng nacelle.
How that translated to the yoke, I have no idea, but on the next leg there was no more vibration so I have to believe that was the cause.
Obviously, I'd suggest a thorough post flight inspection in such cases
Best,
z.