2010 US 5 door stock exhaust, big rattle

[127.0.0.1]

My 10,000 mile Yaris HB has develped a loud exhaust rattle
when I am sitting still and hit reverse, the RPM's drop a bit and a loud
rattle can be heard, either from the flex coupling on the exhaust or a heat
shield.

it sounds like a spring is loose. it is a metallic rattle that comes and goes
on low rpm. doesn't happen when rpms go up. sometimes happens when
I am moving from park to D and standing still. I hit the e-brake and got
out of the car and it sounds like mid-body, not up near engine...

Anyone else had this ? I am taking it to the dealer for an ordinary oil change
and tire rotate, and was gonna ask them to check out the rattle.