It's usually no need for concern.

Try this...

Find the servo that's buzzing and tap it's control surface in each direction. If the buzzing stops, don't sweat it, the engine vibrations will do the same thing as the tapping you did.

Not to mention the fact that if any control surface has any trim applied to it, the air moving over it will also place a load on the servo - but then the engine is running and it's up in the air so you never hear it.