2004 trailblazer engine HMMMM sound

It is between 55-57 mph and it will go on and off HMMM HMMMM I took the service manager for a ride and he also heard this noise but they have no idea what is wrong. They balanced tires and that did not fix it. Anyone deal with something like this?
Thanks

My 02 makes that noise at 75-80mph.

Okay, you're probably not going to believe this but I have the fix. Mine also had the infamous "HMMMM sound" and this is what I found...with help from the GM Tech Bulletins! If you have the roof luggage rack, which I have not seen many TB's without, there is a specific postion that the rails must be in if the rack is empty. Release the slide locks and place the front rail just back enough to be right above the division in front and rear doors. The rear rail should be all the way back. If you have moved these for any reason and then not moved them back, you will experience this noise. I worked on mine, hopefully it will help for you! Let me know.

Took the cross bars completely off. Didn't help! I just drive 70 or 85 now...:-)

I did notice that the blades were installed wrong. They had the "Front/Arrows" pointing towards the rear! I've replace them with Yakima Rack- easy to remove towers with locks system.

I have a 2005 TB. When it arrived, the luggage bars were move about 16 inches in from both ends. I moved one to the the front, and one all the way back for aesthetics. I have not noticed noise, however, I did notice a lot of tbs had bars set as you suggest, even new one on lot. Also, I have heard hmmm-hmmm sound on by 2002 blazer at 55-60mph when wind hits it right.