ML320 Tapping Sound

I have a 1999 ML320. There is a tapping sound coming from something mounted on the drivers side firewall, not the engine itself. It is consistent about 2 taps per second....
What is it? It appears to be something mounted to the drivers side firewall up high, about 12-15 inches in from the side of the compartment.
Thanks
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How old is the car battery? If it is the original, replace it now.
I am not familiar with W163 cars. I had similar sound (though slower, about 1 tap every 2-3 seconds) on my 96 C280 (W202) from around the motor driven throttle valve (I suspected) many years ago. I never figured out what it was but the sound disappeared after battery was changed. It may be just a coincident but worth checking.
wx4usa wrote:

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The throttle is atatched to whatever is making the noise, so Ill check the battery next. Thank you for your input! Hal
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