Is it better to have kayak upright, inverted or sidewise on roof?

I purchased a used Outback today and mounted it inverted on top of my factory tacoma roof rack. I don't know why, but it sounded like I had a jet turbine while driving on the freeway. I never had this issue with my Revo mounted inverted. Wind noise definitely is an issue. I guess since it is wider kayak?

Try plugging any flush mount rod holders or other tubes with a rag or something to stop the jug blowing effect.

And this: 

If you put a couple of twists in the "open" (the part of a tie down strap not against something) eliminates a lot of wind noise. 2¢

Thanks piski. That makes sense. The rod holders were missing those caps actually. I'll try and plug those up next time