Hi good afternoon, this could be a worn belt, the way its made it will make that sound when you are to one end or the other but should not make the sound when yo are in the normal operating positions.
I would suspect that as the cause of this problem.
But you can remove the belt as you should and check for wear on the underside of the belt and also the platform the belt runs on. Please check that and let me know what you find.
Also if you find the belt is the problem you can get a new one from Treadmill Doctor for a good price.
Please contact them for any part you need on this unit as they carry most of them for this model;-)
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Most customers who use them are extremely happy with their service as well.
Since this may take a little time to fix, please rate my service kindly thus far there is no cost for that. That way we can also remain together until this is fixed you can contact me again after rating simply by replying to this message;-)