I have been reading many posts on this site and a lot of people were saying it was the header heat shield. However, I found my problem to be the passenger side Axle heat shield. Now I am not saying it couldn’t be a header heat shield. In my case it was the axle heat shield.

I have a rattle the doesn’t exist when cruising around town but at highways speeds the rattle would start while accelerating on the passenger side floor board.

There is an issue with the steel wool like washers that are used. They wear away and the bracket is free to left giggle and rattle.

Hope this helps!

here is a Honda diagram and it’s part #19 I am referring to.

https://estore.honda.com/honda/parts...b=B++2101&dl=#

Let us know if that was it

Rattle in my Fit turned out to be a bad Catalytic converter.

Just so happened that I wrote this down.
I first triggered a P0420 when my OEM Cat started having problems at 337,064 miles.
(FYI, I have a 2007 5AT).