2017 Tucson makes dull thud around 15 miles only on initial start up, will not happen again until engine turned off and then on again, dealer said that a lot of 2017 Tucsons are doing this and they have contacted hyundia for help.
Are you sure this is not the ABS pump?
Either way, that is why you have warranty. Use it as you are.
What the OP describes certainly sounds like the normal ABS self-test. Normally, this takes place when driving for a few feet–after the car sits for an extended period of time. It usually takes place at a lower speed than 15 mph, but perhaps the newer Hyundais are different in this regard.