As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
EXPERIENCED AUDIBLE NOISE APPROX 2 HOURS IN TO ACOUSTIC ANALYSIS TEST FLIGHT THAT WAS BOTH HEARD AND DETECTED. THE NOISE RESONATED BETWEEN 70 AND 80 DECIBELS. AFTER REDUCING ENGINE POWER AND DESCENDING TO A LOWER ALTITUDE, THE NOISE WAS NO LONGER PRESENT.