I went out to my garage tonight after about 5 hours of my car not being driven. It was making an odd humming noise coming from underneath about midway down. It almost sounded like the gas purge sound that it makes when I shut it off. Any ideas? I don’t want to take it in just yet because I have no idea how to reproduce it. I turned it on and back off and the noise is gone.

 

If it’s a hybrid, that’s not uncommon for it to make some noises up to a few hours after driving. If your car is the standard NX300 then I will have to defer to others here to respond. I wouldn’t expect the regular turbo version to be making those noises after five hours.

 

The information below is copied from the NX300 technical training manual, and may explain your observation.

Typical set of conditions required to initiate an evaporative emission leak check:
5 hours have elapsed after engine turn-off; **
Altitude less than 2400M (8000 ft) above sea level;
Battery voltage greater than 10.5V;
Engine off;
Coolant temperature = 4.4C to 35C (40F to 95F);
Intake air temperature = 4.4C to 35C (40F to 95F).

** NOTE
If engine coolant temperature has not fallen to 35C, the elapsed time after engine turn-off is extended to 7 hours. If still not less than 35C, the elapsed time is further extended to 9.5 hours.

 

I have caught both my '17 NX200t and '14 IS250 do this several hours after driving - normal emissions system activity.