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?