I ended up removing my Curt hitch today... I removed the rear bumper and found that the hitch was installed properly and not sandwiched, so with no hope of fixing the occasional rattles I took it out. I have no idea how the exhaust came into contact with the mount since theres at least an inch of free space in between the two parts. But some marks on the exhaust mount confirm that sometimes they do come into contact.
If someone in the Montreal area wants it it's up for sale.
2009 128i - may 2009 production - SGM - sport - premium - usb - ssk - perf steering wheel - roof rack - debadged