The only difference I can tell is that the faster I go, the more consistent the rattling gets. So rather than it be every now and then, it rattles much more frequent.
This morning on my way to work, it only rattled a few times in my 25 minute drive, but then again I was under 45 most of the time. During most of yesterday, however, I was driving from Tallahassee, FL to Miami, and going above 65 to 70 95% of the time. The rattle was more frequent and obvious.
I will test to see if it in fact is the tail pipe.