I have an auto 08 G37 coupe when I rev between 1700-1900 RPMs at around 5-20 MPH this strange noise comes from the exhaust.

Any higher RPMs or faster than 20mph the exhaust sounds perfectly normal.
Also sound only comes in during DRIVE, not in neutral/park.

I am just wondering if this is normal noise for low speed low RPMs ??

*EDIT*
Sorry for bad first video! I have redone it with my phone taped to my license plate lol... so if you hear clanking it's the license plate shaking with my phone.
Recorded at night in automatic drive happens in 1st, 2nd, 3rd gear... 4th gear I am going too fast and the sound doesn't seem to exist. Can't replicate sound in Park/Neutral ! Only in DRIVE !

Car drives normal... no MPG problem, no shifting problems.... but I did have the ECU changed out last year for the high pitch frequency noise if you know what I'm talking about. It was a high pitch noise only a few people can hear (unfortunately me) when you go past 1,000 RPM. And I don't remember if I had this exhaust sound before/after the ECU change. I actually didn't really pay attention to this sound until my friend brought it up to me.

http://www.youtube.com/watch?v=4BycG6oqr_E

 

Had the same problem, took it into the dealer. They said my muffler baffles were rusted from the inside?? Not exactly sure, not much of a car guy. But they just replaced my muffler and it sounds good.