Ok to finish the thread. I feel dumb even admitting what the sound was, but Brian's post was right, "operator error" lol
. It was the shock on the idler pulley. The bushing to bolt wasn't tight and it'd rattle a bit while coming down to idle or at times while idling. I put a rubber washer on the bolt between the shock and engine block and it's silent. So it wasn't the IP, just a rattle of an old car. The stop button still makes it's sound when pressed but I'm guessing that's normal.
On another note... I was at a diesel injection shop a few weeks ago having my injectors pop tested and they had some pump on a bench and they were spinning it on a machine.
That you could hear. Without the engine noise the IP spinning for a bench test at different RPMs sounded just like the engine. I think it was a 4 cyl box truck IP because it was a quieter version that was resemblent of a small Isuzu or Mitsubishi type engine running. Sounded pretty cool actually! Wouldn't mind an IP in a case somewhere on display in my garage. Cool souding little thing! Pretty neat.