Noise
Tom,
Is this noise with the top up or down?
What level is the bass set on the stereo?
Sometimes the bass level is just right at certain speeds due to the speed activated sound intensifier (volume goes up as speed increases to overcome the increased noise levels) in the stereo system to create a low intensity groan, moan or vibration you may be experiencing. Mine would do this at certain speeds, and I found this out by turning the stereo off while it was creating this noise (it went away), and tuning it back on while at constant speed (it returned),
Give it a try to see if yours is the same thing.