Oh the horror, what happened to my 340...

Very hard to tell by listening to a video. However, I didn't hear anything banging from the bottom end, so I hope it's nothing serious. Almost sounds like somebody installed a 246 duration @ 050 when you weren't looking. Is it popping out the tail pipes??

No popping from the pipes. Under the car sounds like the same thing I'm going to drop the oil pan and pull the valve covers to see what I see.
It was built with Comp 822-16 lifters and comp 20-227-4 cam (xe275) If I recall the duration is 231 and 237.