2015 RX doesn't have DI...so yes, it's not normal for it to do it...since it doesn't have that system.
Maybe you are getting detonation from the type of fuel being used.
If that's the case, the dealer will advise to change fuel for a few tanks and see if the noise improves or goes away.
Was there any other change with the vehicle at 15K?
I wouldn't be ready to jump on the "Lexus camp" at this point to put the blame...lots of other things to look at first.
As a side note, ALL DI only engines make that noise.

 

In many of the older port injection Toyota V6 engines the tick usually comes from the fuel vapor feed area. Forgive me I can’t remember the name of the part (fuel vapor feed sensor) maybe, but it is often described as a sewing machine sound. This is a normal noise that will arise after some miles and age, in the case of the ES300. Many thought something was wrong plenty of times with their Camry and Avalon’s because of this noise, but it was in fact normal for it to sound as such, replacement of that part will make it quiet again, but know it’s no malfunctioning, it’s just noisy, but at times it has failed, in which case you’ll know.