I have kayak racks (Mako and Hully Rollers) and bike racks (raptor?) on my roof rack, so I can carry bikes and boats at one time. I only experience the noise when the crossbars are empty. No noise when the kayak racks are on. I was suprised it made a difference, I was considering one of those air deflectors but instead I put the kayak racks back on = no noise.
I did meet some guy who actually took the racks off the roof completely including the crossbars and towers and put them inside his car, even when we were just away from the parked cars paddling for a few hours. Personally, I done have the time to keep installing stuff everytime I want to use it.
1985 Nishiki International Touring Bike, 1992 Vitus 979 road bike, 1996 Bianchi Premio road bike, 2002 Thin Blue LIne CO2 mountain bike, 2007 Rocky Mountain Sherpa touring bike, 1964 CCM roadster, 1959 CCM Motorbike, 2002 KHS FXT mtb + more to fix!
I have Yakima round cross bars on the factory rails on my 04 Impreza. They create some wind noise, it seems to be more on the passenger side of the car for some reason. I haven't tried different cross bar spacings but I think they would make a difference. I'll probably end up getting a fairing, it just seems like a lot of money for a hunk of plastic.