This particular motor has a rather complicated internal cam chain tensioner. (before I get flamed, yes, it has an external belt, this one keeps the adjustable cams in time). This tensioner can fail, cause a bit of rough running and what sounds like rattling inside the engine, especially under low RPM’s and heavy load (like, say 4th gear, at 20MPH, and you put your foot down). Almost like pinging, but not.
I can’t diagnose it over the 'Net, but this part may run ~$600 (there are a couple different ones, $600 is the expensive one), and replacement labor will be about another $600. The top of the motor needs to be ripped down to get at it.
I don’t know for certain this is the problem, but it’s one possibility. Wish I had a better idea for you, and i hope this isn’t the issue, for your wallet’s sake.