My recirc A/c button goes off with a knocking sound inside the dashboard and the blower also slows down very much almost like it is off and then slowly picks up speed.this cycle continues. The outside temp is very hot like 46C and when the blower slows down and the recirc button goes off it is quite warm inside.
Is this difficult to fix.

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300CE 1989 Euro Model.
ML350 2011.