turn your hvac off and see if it stops I had the same problem with mine and my brothers x3 you could feel it where you rest your foot . Replaced my heater valve with a used one never had the problem again.

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-e24 m6
-e32 740/5
-e34 525it
-e83
-e83 Msport (gf)
-e30 325ix (gf)