On the W114 which may or may not be the same as your 240d, the air flap just above the throttle pedal has a switch on it to redirect the current to the AC fan or heater fan. The clunk may be the flap moving but not making the switch.

Michael

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Usta haves '69 250/8, '76 280C, 1971 250C 114.023, 1976 450SEL 116.033
Current have, 1983 300SD 126.120