Treadmill repair

I have a Healthrider Treadmill 900 HRC that is 4 years old. Just about everything (belt-platform-drive roller) were replaced. HERE IS THE PROBLEM. It makes a sound when the left foot strikes the platform. It is not like rubbing sound. It sounds metallic. I checked if the board is hitting something- No. I checked if the belt is hitting something- No. I checked if the board is cracked. No. This is actually a new board. I checked and rechecked. The sound comes when the board is pushed by the foot (on the left side -where the drive motor is). You can also create the sound by pushing with hand in that location. I am suspecting that the metallic frame has a crack in it. Has anyone faced this problem. How to check for this and how to fix it?
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