It sounds kind of counter-intuitive, but I've found that my roof rack makes a lot less noise if I have foam pipe insulation fastened around the load bars, versus just the naked load bars. With foam I get a dull roaring sound up to about 70 MPH (didn't go faster). Without, I get load howling/humming starting at 25 MPH.

I don't know if it's the larger, rounder cross section of the foam, or if the soft foam absorbs some vibrational energy. But it's a cheap way to cut noise. I only do short hauls with my rack, so I have no guess on impact on fuel consumption.

This is on my Inno rack, which has rectangular load bars that are the same shape as Thule rectangular bars.

 

I have used foam pipe insulator tubes on roof rack bars for years, but not for noise reduction. The foam keeps the roof load objects like ladders from scratching the paint off the bars, and keeps the bars from denting softer load items like lumber, plywood, moulding and other construction materials.

 

Me too - I normally have foam covering mine. I find the foam is more versatile, even for me kayaks. I was setting up a second set of bars that I got at a garage sale which was the only reason I ran without the foam.