is it a turkey sound when you shift?
I was thinking more of a main transmission bearing. I don't know how a mufler leak could resolt in a loss of significant power.
Maybe cats gone (loss of power) and vibrates that stupid plate (noise).
1) Open hood. Proceed to 2.
2) Look for turbo. Proceed to 3.
3) Is turbo present? If yes, proceed to 4. If no, proceed to 5.
4) Your car has a turbo
5) Your car does not have a turbo