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That "diesel" noise is the gasoline direct injection system that all late model Corvettes have. I hear it with my 2017 at idle and it is more pronounced when the hood is open (of course). I don't hear it when driving down the road at all, even with the windows open. My girlfriend's 2019 makes the same sound too. Both are non-Z51 Stingray coupes with the LT1 engine. My car has the 7spd manual, while hers has the A8.
Regarding the plug-in "Range" device, spend the money and get one if you have an automatic transmission. It will keep the car from going into V4 mode. With the M7, just don't drive in "Eco" mode and the car will always run on all eight cylinders.
Your control solenoid on the high-pressure fuel pump is on the driver's side rear of engine where the noise is coming from. And adding a Range devise will have no impact on reducing pump/director injector taping sound.
However if you have an A8 trans, wouldn't be a bad idea to get one.
Your engine sounds just like my own Z51, H. - I'd guess it's normal. It might be a funny sounding engine, but it's got the torque of a LS7 and the fuel mileage of a V6 Impala. I'm kind of happy with mine, funny noises none-withstanding.