Radio Riddle

[Aviator305]

I am the noise on this audio file. I am hooked up to a MAC 1700 radio that was recently removed during a visit to an avionics shop. I became evident after my pilot turned final after a 2 hour cross country, and I fade away when engine RPMs are up above about 900 RPM. At lower RPMs, the alternator switch had no effect on me. My pilot could not get rid of me on the ground and thought he was going to have to spend many AMU's to track me down. I went away on my own after 3 more hours of flight. What am I?

 

[N33GG]

 

[Unit74]

 

[kf6iat]

 

[sierrakilo99]