Thanks for your informative answers.
Yes, it did sound like an electric strimmer somehow. But suddenly like a very loud strimmer
I didnt realize a level off, but well, guess at some height above the clouds is hard to say whether the aircraft is levelled off or still climbing, since the noise is pitched up a bit anyway.
I heard that in flight, one thrust lever is always set a little bit more forward than the other, because of passenger comfort reasons. Could an increase in sound also be caused because both engines are running on exact the same thrust?
Am going to have a trip on an A330 soon, are there also any specific noises for this aircraft to expect?