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How to fix exercise bike? - my exercise bike clicks as i pedal

We have an exercise bike Reebok, one of them with the computer in front of the console so that you can adjust the tension, heart rate monitoring, etc.
If the pedals come into it, and start really bad and hard to pedal. If you have a pedal a little and then back again, the problem for a few seconds is correct, then the problem is back.
Does anyone know a way to solve this problem? The base is a plastic coating hard to see what is the problem? Thanks