Background rumble

[veshtd]

Hi All, I recently purchased a 225 Cup with about 20K miles on the clock. All is well except a low frequency rumble that is noticeable when I'm on smooth road surface doing about 40mph. I'm no expert but it sounds like either a wheel bearing or driveshaft? Although the sound seems to be coming from the rear. Just thought I'd post first prior to booking it in under warranty. Thanks.