Front Runner Cheetah rumble.

[stewmair]

I have had this excellent roof rack of 6 years. For some reason, possibly harder tyres, a rumble became evident at 80-100 kph.
Fixed by bolting a length of 6x25mm aluminium strip underneath along the midline of the rack. Used 25mm ss allen keyed bolts with ss Nylock nuts. The bolt heads sit out of the way in the top slots of the crossbars.
Surprisingly this worked and also markedly reduced the rumble at lower speeds on rough bitumen. Happy result!

[ADMIRAL]

Photo would be nice.

[stewmair]

Photos to come, pronto, if I can do it!

[stewmair]

hope these work and are sufficient.http://www.aulro.com/afvb/attachment.php?attachmentid=81878&stc=1&d=1407653592http://www.aulro.com/afvb/attachment.php?attachmentid=81879&stc=1&d=1407653592
and they did!!

[ADMIRAL]

Ok......... So the alloy rib runs lengthways down the rack centreline . Not sure why that would make any difference, but it would stiffen the rack. I have the later model rack, and have added extra cross rails. It may not be of benefit to my setup

[stewmair]

Unless the rails all vibrate together it probably doubles the natural frequency and markedly reduces the excursion with each beat cycle. If the all vibrated in unison it would be stiffer as you say.
Hope you can fix your problem as simply.