Just a thought here. Are you relating prop-tip noise frequency with engine rpm (2250-2450....it seems like it) or did you actually measure the frequency? If the former, note that sound frequency is in Hz, or cycles per second, and the prop noise would be on the order of cycles per minute. Also, the prop will have two or more tips which would change the calculations. A two-bladed prop at 2400 rpm would produce an 80 Hz sound (not even considering the harmonics or the exhaust noise).
I could be all full of it and usually am but I wanted to draw attention to this in case it applies and you hadn't considered it.

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1983 M-B 240D-Gone too.
1976 M-B 300D-Departed.

"Good" is the worst enemy of "Great".