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If you experience a starter motor noise on Porsche 911 and you have the feeling that it is close to a loud noise, it is conceivable that it is the starter motor gears that are at the origin of it. It doesn't sound like that though from your description, I would just check the wiring for the tach itself, and for the engine speed sensor which goes from the engine to the DME wiring harness. I perceive noises from the starter motor of my Porsche 911, what are the causes I feel a loud noise from the starter of my Porsche 911. However, if you feel & hear the engine actually revving up sporadically as the tach is indicating, then perhaps the clutch is slipping. If you are hearing more noise from the engine area it may be time for new engine insulation. Engine compartment insulation helps control the heat and noise created by the engine. It is not likely to be a reflection on the health of the motor. Porsche 911 (1974-1989) - Engine Compartment Insulation and Seals.
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If it doesn't happen again I wouldn't be too concerned about it, but I would make sure to get an oil change with an approved oil, and have them check & clean the magnet on the drain plug just to make sure there are no metal shavings in there.The tach jump is probably an intermittent wiring connection or ground somewhere due to the effects of time & weather. This is more likely if it was a hot day, and we know the oil cooling is not ideal for high rpms at low road speeds, as you would have been doing, the car's cooling systems work best at higher road speeds with more air moving through the systems. It is possible that the oil got stressed and didn't hold up 100% under the increased load, you could say it might have "thinned out".