Soft knock sound when heater on. Anyone else?

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2018 CX5
We have 22K miles on our 2018 CX5. Recently noticed a very soft knock sound coming from the engine bay when the heater is on. Goes off as soon as I completely turn off the climate control system. Sound only happens once a month and then goes away. Heater and AC seem to work fine. Sound is so soft can only hear it when car is not moving or moving less than 5 MPH.

Any ideas?
 
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2018 CX5
I'll try to play with the controls next time it happens. (drove 80 miles today and nothing). But it would seem not to be a common issue from the lack of responses here.
 
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2018 CX5
Played with the heater/ac controls last time sound appeared. No change with AC on or off. No change with fan speed. No change with changing fresh to recycle air. Only when I completely turn the ventilation system off is when the soft knock goes away (also has no connection to engine speed). Very odd.

Anyone know what part of the ventilation system could make an intermittent soft knock?
 
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