Perhaps you are not listening close enough. Sometimes water or moisture will get into one of those sealed sensors and that would be end of that one. To hear the clicking, I used a metal screw driver close to a sensor and sure enough, the clicking is there.

Also is the long beep from the front or the back? May help you further isolate the issue.

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2017 Tesla Model X
2014 Porsche Panamera-S e-Hybrid (sold)
2012 Porsche Cayenne
2001 X5 3.0 (sold)
2003 Carrera 911 Cabrio (sold)