ÜberMpower

The Vanos Rattle sound is described as like marbles/pebbles in a can or like a diesel engine, which can get worse as the car warms up from a cold start!

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This vanos, like most vanos designs, can develop a rattle.
The vanos rattle is caused by wear in the variable valve timing helical (slanted) gears. These gears are found on the camshaft, camshaft sprocket, and splined shaft which attaches to the vanos.
The helical gear wear allows the camshaft to have lash movements that engage the splined shaft axially. At certain RPMs these movements resonate and engage axial play (free space) and cause associated components to hit and rattle.
The axial play that facilitates the rattle is found on the helical gears and the vanos piston bearing. Replacing the helical gear components is expensive and the gears will wear again and the rattle will return.
Removing the vanos piston bearing axial play significantly reduces the rattle to the point where it’s barely heard or not heard at all in the passenger compartment.
Performance cams like the cams on the M3 or aftermarket performance (Schrick) cams create stronger camshaft lash forces and thus are more susceptible to causing a rattle.

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Disclaimer: Follow Procedure at your own risk

If in doubt speak to Mr Vanos

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"M Abteilung Über Alles", Translates "M Division above all."