<E30> Thunking Noise form Heater Area

["Stroes, Gustave" <Gustave.Stroes@xxxxxxx>]

Digesters,

I recently replaced the heater core and heater valve on my 88 M3.

The other day I turned my thermostat about 30 degrees clockwise form the off
position.  Just far enough so that the heater valve solenoid should be open.
I was driving in town, with many stop and go's.  I noticed a "thumping"
sound coming from behind the center console.

After some experimenting I have discovered that the frequency of the thumps
is related to engine RPM.  The thumping starts at about 1000 RPM's and
increases in frequency until about 2800 RPM's at which point it ceases.  So
driving on the freeway with the heater on there is no thumping.  If I slowly
decrease the RMP's through the range from 2800 down to 1000 there is no
thumping either.  But if I come down to idle and then slowly bring the RPM's
back up again, then the thumping manifests itself again.  The relationship
between the frequency of the thumps and RPM is very repeatable.

It sounds like some kind of valve opening and closing (the heater valve
maybe?).  It is very peculiar and I am not sure where to begin looking.

Does anyone have anything to suggest?

Thank you,

Gustave Stroes
88 M3
Redondo Bch, CA

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