Noise from jet engine - normal or not?
On our citation cj1+, the right engine made sort of a humming, low tone when we were on cruising altitude FL400. This tone was audible for 2-3 minutes and then disappeared. All engine displays were normal, no vibrations whatsoever. The tone would come on again now and then for 30 seconds to 1minute. The tone was not audible when we had to level-off on several occasions when descending. BTW, the tone was conspicous enough to make our passenger ask us if everything was alright with the right engine. Besides the obvious thing to do (call maintenance), I would appreciate any input.
Cheers
Cecco