The Eastern Diamondback looks to be the closest match, although the colours and markings are still not quite right. 

The pics I Google'd on the Eastern Diamondback all look grey-brown-black whereas the one we saw was reddish-brown.  And the diamonds were a very different shape.  This is so odd...  But whatever it is, I'm pretty sure now that it's some kind of rattler that was missing it's rattle.  It's gonna have to go.  I don't want a poisonous snake hanging around.  I don't like killing anything, but I'm not taking a chance with trying to trap it.

Thanks so much everyone!  If you can think of anything else it might be, please let me know!  I'll try to take a picture if we see it again.

"You don't know what we can see
Why don't you tell your dreams to me
Fantasy will set you free." - Steppenwolf