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I don't know, I would tend to side with the theory that the car engine sound at the specific RPM and power output matches the resonance frequency of one of the house windows. Resonance can creep up in many phenomena.

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But the engine speed doesn't change until you lift the clutch. So if this were due to the RPM, putting it in reverse while stationary surely wouldn't do anything in particular.

If it’s an automatic, putting into reverse will change the engine RPM.

Fuel mixture changes as the engine heats up, changing the sound

Not in my opinion because this would have been a problem before with other cars and alarms. Or motorbikes.

Without knowing the details of the alarm system it is hard to have an educated guess.

I've owned a number of Subarus and let me tell you, even the non-performance models have a way of shaking things inside the house that you wouldn't believe until you experience it. My STI was capable of being loud, but oddly enough it was my wife's Outback (nearly silent if you stood next to it) that would shake pictures on the walls and make them start tapping.

Very true. My '13 WRX shakes pictures when I start it up in the garage, I have to immediately pull into the street so I don't piss anyone off while getting Google Maps and Spotify set up before I drive.

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