I like the Thule rack on my PT Cruiser a lot, but I found the wind noise to be very loud when the kayaks are not in place. One option is to take the rack off when I’m not using it but that’s a pain in the butt.
I think most of the noise comes from wind hitting the flat face of the rack as I drive, so I figured that streamlining the rack would help. I tried several options and ended up with a pretty simple one which lessens (although does not eliminate) this noise.
I bought some half round wooden molding (3/8", I think, but measure your own rack) and cut two pieces slightly shorter than the rack bar. I simply held these in place with black nylon ties-- those little ratchet lock thingies. One piece in front and one piece in back of the rack. Then I just spray painted the whole thing black. If I had it to do over, I’d paint the molding first, but I was too lazy to do this 
This reduces the excess noise quite a bit. I only did the front bar, since I think the rear one is back far enough that I’m not really hearing it.
I do this…
for the Thule rails on my Jeep. Pool Noodles! They have to be the hollow ones though. Since you have cradles or something like that. Cut the noodles down to the smaller sizes needs and slice them down the middle and pop them over the rails. Cut the wind noise WAY down to nothing. I tie my kayak right to the noodles. It’s an older plastic boat and has been very effective as a means of transport for a few years now.
As an added benifit, you can spot your vehicle in a crowded parking lot.
spotting my car…
LOL- spotting my car is not a problem. It’s an electric blue PT Cruiser, and with a pair of yellow and orange Pungo’s on top it stands out in the crowd fer sure! I like the noodle idea.
Spiral-wrap bars with rope
If you wrap 1/4" or larger rope around the front bar, with the coils about 2" apart, most of the noise may disappear. It seemed to work on a Yakima rack anyway. The pool noodles, or smaller pipe insulation foam, also seemed to work.