Join Date: Oct 2005
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I have had the same problem with my 1.4 16v Megane. The symptoms being identical, i.e. when switching from external to internal air source (for the Air Con) there was a knocking noise as if something was trying to engage - and not. I guessed it was connected to the heater 'cos when getting into the car and closing the door, the resulting gust of air made (what I assumed was) some kind of flap "knock" inside the heater. I had this cured at a cost of £160 last summer (2004) by a main dealer (after 2 attempts over 3 days) and would you believe it it's gone again although I'm not getting the knocking this time. The really bugging thing is that I rarely use the Air Con, and the internal airflow option only gets used in traffic jams.
Currently in Longfellow's garage: