Yes, this problem is very common! There is only one solution I recommend for the perfect look and actually works. Rigid produces a special product made for the roof mounted light bars. They are called Harmonic Resonance Dampers (or E-Series Resonance Dampers). I have them installed on my Rigid 50". It's amazing the difference it made with noise.
I don't know how you buy them because they aren't on their site. I got mine for free from a vendor that does a lot of business with Rigid. Maybe you can call Rigid? You will need three or four for the 54" RDS.
I actually have 3 of the dampers installed and it still has some minor whistling. It's not to bad with the radio turned up, so I guess I'll just deal with it. Other than that this thing turns night into day.