sounds like a bit of over run as the engine breaks itself through downshifting, nothing too serious, just keep it monitored. Like others said, likely part of the break in period.
P.S dont forget to get on your Viking a little during break in, the worst thing you can do to it is baby it when its brand new. You need to set the piston rings and tighten up the cylinders, only way to do that is some hard running. Just try and keep it on the flat because the rest of the driveline does need to be treated gentle for a bit...