Altitude may have something to do with volume, the lower to the ground, the louder it gets.
The house I grew up in was under the small-plane landing pattern for the city airport, and an Army Reserve Huey company was based there. If they were climbing away from a take-off, it was a sedate bup-bup-bup-bup. If they were descending for a landing, it boomed a lot louder, echoing off the houses. If they were really low (I can remember on at least one clear day in my side yard, being able to see the pilot's feet on the control bar through the nose glass), the windows rattled, the house shook, and the freestanding bookcases in the living room would rock back and forth. BAMABAMABAMABAMABAMABAM! It could drown out the lawnmower I was pushing.
We felt a small earthquake once and I saw the same bookcases rocking, but couldn't hear the chopper.
Strangely, I can remember once they landed one of the choppers at my elementary school for a special Bicentennial event (remember 1976?), and I don't remember the bird landing in the same schoolyard being as loud as a few of the times they came over my house.
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My Twilight claim to fame: I ran "Allegheny Uprising" at Allegheny College, spring of 1988.
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