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Yes you described drone quite well. From what I recall it the natural harmonic sync which occurs at a certain RPM range. I believe our cars 2500-3000. Usually highway cruising speeds. Annoying to some and tolerable to others.
PSE in 987and newer cars has the crossover valves electrically actuated via SPORT button on the lower center console.
PSE in our cars are vacuum actuated and always active unlike the 987 cars. I believe ours only operate under throttle which produces vacuum.
I could be wrong someone can correct me
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