I just bought my 4th 300 series and I've never had an issue with my heat/fan before (beyond replacing a blower motor or monovalve diaphragm). So I'm totally lost

I have heat about 50% of the time.
When I use the fan, half the time I get outside air temp, half the time I get heat.

The primary issue is when I turn on the climate control on high it sounds like a small aircraft/leafblower. It’s a strong vibration. (dead center, not blower motor)

The speed and volume of the sound/vibration increases with acceleration, decreases when decelerating.

When I turn the fan to low, there’s no sound at all.

What in gods name is this?

thanks
bates

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1985 300D Surfblau "Blueberry" 250K R.I.P
1984 300CD Manila Beige "The Banana" 238K R.I.P
1984 300TD Cypress Green "Olive" 390K (M.I.A.)
1982 300D Orient Red "Steak" 195K
1985 Euro 300TD Lapis Blue “Pancake” 200k KM
1982 300D Light Ivory “Butter” 183k
1984 300TD Black “Pepper” 55k