Sounds like bad gas to me. Does it sounds like marbles or ball bearings in there? If you got some bad gas then you're causing predetontation. This is what happens to me when I try to run 87 on a engine that only takes 91 or better
Check this out by running that gas out and get some known good gas from a different station. If it goes away then you know what it was.
I doubt it's a exhaust leak since it doesn't happen at idle. An exahust leak usually is really noticeable at idle.