The outflow may become sonic during a small part of each revolution but that should not create what you are describing. A 14,75 mm end cone restriction should be good for over 22 crankshaft-HP, so no problem there. Other noise-contributing factors can be a high exhaust timing, a late ignition timing and a low compression ratio.
And of course an absorbing-type silencer that is not absorbing because its stuffing is either too tightly packed, saturated with oil, or gone with the wind.
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