Advice for a 23 new car owner please

Hi everyone, I just bought a 2018 VW Tiguan and I need advice. I noticed harmonics around 50-60 mph and they replaced the rear differential fluid but the sound is still there. I still have my 60 day warranty from the used car place but if I bring it back again are they going to just dismiss my car after they "check" it. My biggest worry is there is something going on with it and they don't care and it goes wrong after my warranty is up and I have to pay a lot of money I don't have. Any advice?