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Did I park on a squirrel?

I have 2001 Acura 3.2CL type-S, when I turn on the key, even if I don’t start it, it makes a rythmic thumping noise for about 30 seconds. The first time it happened I had just pulled to the curb, and it sounded like a squirrel was thumping it’s tail against the undercarriage of my car. It happens intermittently while driving, but does not change tempo with speed. Any ideas?

Is it possible that the noise is actually coming from behind the dashboard, rather than from underneath the car? I ask this question because a possible cause of this type of noise is a malfunctioning “blend door” in the HVAC system.

Have you noticed any problems with either the temperature of the air from the HVAC system, or problems with directing the flow of air where you want it to be? Test all of the modes of the HVAC system (floor, dash-level vents, defrost, bi-level, heat, A/C, recirculate) to see if there are any abnormalities.

If it doesn’t change with speed or engine RPMs and it only happens on startup it makes me wonder if it’s something like the ABS system self testing.

Have you checked for any stored codes?