I was hiking one day, several years ago. Upon approaching a rocky clearing, I began hopping from rock to rock when a bronze-colored flash caught my eye. As I neared closer to where the "thing" appeared, I heard a distinct "rattling/buzzing sound"... Instinctively, I high-tailed it in the other direction. I had never heard a rattlesnake before, but I did not need anyone to tell me that was what it was! A little while later, after I had recovered my composure, I ventured again closer to the last sighting... Sure enough, It was a rattler.

In other words... I have never heard a chain-slap on a 117 engine, but I think I would recognize it immediately... And I would expect it to sound like you described.

I think I would get a second opinion rather soon... maybe even tow in to shop rather than drive.

Tow cost... $70 bucks
Engine replacement... $7000 bucks
Piece of mind... Priceless