2009 Pontiac G8 RPM fluctuation

I have a 2009 G8 that recently started exhibiting some fluctuations in RPM in the 30-45 mph range. As the RPM's fluctuate, often 200 up or down, a humming noise emanates from engine/transmission area. It almost sounds like when you drive over those noise strips put into the highway to prevent cars from traveling off the road. Most commonly noticed in the 1600 RPM range, but has occurred below 1500 as well. Not noticeable at higher speeds. Have you ever heard of these symptoms? It is a V6 with 50,000 miles on it.