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Sorry for the delay and thank you for your patience. My name is Michael. I will be helping you with your question today. Please let me know if there as any additional information that you feel might be pertinent to your question. If I don't hear back from you, I'll provide the best answer I can based on the information you've given so far. You will still be able to reply with additional information or questions at any point during our conversation, and I'll revise the my answer accordingly.
This could be several things. I understand you have ruled out the belt. What I would suggest doing is opening the machine and testing both the main motor and the incline motor. To do that, apply 12-18 v DC directly to the terminals. Then you can find out if the motors themselves are causing the sound.
I would also check the belt for wear though. When on an incline the can be more friction between the belt and the deck and if the belt is worn, it can make all kinds of sounds. If neither of those help, let me know and we can go from there.
Ok, so just the pressure on it when on an incline with nothing running causes the same sound?
What if you have it unplugged when you do that?
OK... sounds like it would have to be something related to the frame - or possibly the incline motor, even though it's not running at the time it's still supporting the weight. Please see if you can locate what part of the machine it might be coming from.