We use 2 Thule 591 roof carriers on some whisbar aero roof bars, very good combo IMO. Steady and very easy to load, you use two hands two lift the bike on (fork and seat tube) then they have a single handed securing mechanism so you can still hold the bike with one hand as you secure it.
Unlike some bike racks that need 3 or 4 hands to operate.
Oddly they whistle with no bikes on but are quiet when loaded, but as Mike says thats very specific and some duct tape would fix that If i could be bothered.
They'll limit your speed to 80 tops else they get a speed wobble on. With two bikes on its helps if you cross bungee between them to help steady the handle bars.
E63 AMG here at last!