Gas engine sounds like a diesel engine at low idle?

Hi folks.
I have a question about my grandson's car listed above. It sounds like a diesel engine at low idle but it's a gas engine. We have taken it to two different shops, but they can't figure it out either. At low idle it really sounds like a diesel engine. But if you touch the gas just a tiny bit the noise goes away. Standing in front of the engine it's hard to tell if it's coming from the left side, right side or from the top. We are not getting any error codes or warning lights on the dash. The car seems to run fine. No smoke out of the exhaust, nothing leaking, etc. I'm attaching a short video of the noise. Any thoughts or ideas would be greatly appreciated.
Thanks
Phil