Does it sound inside car?

This may sound completely daft, but check your recirculation motor on the heater box that activates the re-circ flap behind the glovebox, they can fail and cause a knocking.

Little black motor with 3? phillips screws that are a twat to get to.

Unplug it and see if noise stops.

The gears slip inside and can make a hell of a racket.