2006 Chevy Monte Carlo Engine noise rattle pinging diesel s

Car started making engine noise, sounds like a diesel engine when I accelerate.

Here are the details:
2006 Monte Carlo SS; 5.7L V8 with 86,000 miles

This week I brought it into a dealer to replace a failed throttle body and fuel pump. (Car would not start) They replaced those and did an oil change.

The next day I notice a bothersome engine sound. Sounds like an engine rattle, a lot like a diesel engine. But it's not constant, only intermittent. I can only cause it to make the sounds sometimes. Only while driving, not in park or idle.

I've narrowed it down to this:
While driving at about 40 mph, I let off accelerator and let coast to low RPM, then accelerate and the sound sometimes shows up, kinda quiet but noticeable
OR- If driving at about 40 mph and I almost floor the accelerator to reach 3000 or 4000 RPM, then the rattle/pinging sounds again, much louder and very noticeable

I have never heard this before, it just started the day after I left the service garage.

Also note: I always use 91 Octane (the highest I can get in Vegas)
I have always used 5W30 oil, but NOW it has 10W30 oil from the dealer.

I brought back to dealer, but they want me to show sound to a tech during a ride. Not tech was available, so I need to bring back whenever I can get a day off work, which will not be any day soon.