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What does the engine do when you drive to 6000 rpm, let it drop to 2000rpm, and then get back on the throttle?
If the engine is starved of fuel under no load, then it will be very clear when under load. Do you have any codes?
For the humming, I think a good start (and the only real easy and cheap test you can do) is to remove the drive belt and listen for the same humming sound. If the sound is gone, then it is one of your engine accessories making the noise. If the sound is still there, well...then, its going to get interesting.
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