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Depending upon your exact model of elliptical there could be various problems causing what you are asking about.
If there is one place in rotation where the noise occurs, then position the machine's rotating parts right in that position, then try to lift or otherwise move abnormally, the rotating part(s). (You might need someone else to hold the machine in that position while doing so.) If that reveals the looseness, then you might have more data as to where the problem is.
I hope it isn't a broken bearing race. Such clunkings that don't seem to come from anything being obviously loose, can be owing to such a cause.