Something is in duct that rattles with the heater's high fan

Do you know which part of the dash this noise is coming from? If it is close to the blower motor, I would try removing the blower motor and see if you can feel around in there and get it out.

Otherwise, I need to know which duct from the options below you think it is in and we can get the process for it.

However, I attached the process for the removing the blower motor for your review.

I believe it is the left defogger duct, when I place my hand from bottom I can feel the vibration. I removed cover (right side) connected to it (3-5.5mm screws) cover still won't come off, feels like upper left corner holding.

Looks like there is a screw in the upper left corner. Take a look at these from the manual.