[ischmitz]

First off your message is getting "lost in translation" a little bit: The German word "Tachometer" refers to the speedometer (showing car speed). It is driven off the transmission via an impulse signal. If the readout drops while the wheel speed stays constant you might have an issue with system voltage changing.

The American word tachometer refers to the rev. counter (engine speed). You seem to indicate that your speedometer drops by some value 5 - 7 km) when the noise occours, correct?

Thinking out of the box could the periodic noise be your AC-compressor kicking in with a bad bearing or similar. Maybe that in combination with some poor GND causes your board voltage to change. The compressor uses a magnetic clutch that can either be bad (drawing too much current) or simply bog down the board voltage due to issues elsewhere......

But then agian it is really difficult to diagnose something remotely. A good mechanic familir with 911 should be able to narrow things down pretty quick especially when you can provide solid observations of what happens.

Good luck,
Ingo