What age?
If the N52 engine, it can either be (or both) the valvetronic gasket or valve cover gasket.
The valvetronic gasket can leak and fill up the voids around the middle 2 spark plugs, then it drips over the edge onto the manifold.
The valve cover gasket can leak from anywhere along the lower side, and then drips onto the manifold. Both very common.
With the engine cover off check inside the voids around the sparks underneath the valvetronic motor.