Car topping

Now that the truck is gone (until I buy another) I'll most likely be mostly car topping the kayak.  I have a '97 Camry.  Just did a test drive and everything was pretty secure, but the lines on the front of the car fluttered pretty bad.  I was thinking of going to round rope on the front with the flat ratcheting straps at the back to take up the tension.  But wanted to see what the folks around here have had good luck with.  There's no roof rack on the car, at least not yet.  I've got a moving blanket between the kayak and the roof right now.

Were your straps flat?  If so, they will "flutter" and hum badly...
Try putting a twist or two in the strap...it will help with "flutter"...

I forgot to the other day, it sounded like I had lost a wheel and was dragging my axle...it was very loud...
Stopped, a quick twist, and problem solved...

The twist in the flat strap is sometimes referred to as the "truckers twist". Notice all the wide flat straps over a big load on a semi with trailer and they will have a twist in the strap to get rid of the vibrations. If it is not twisted the straps can fray and fail, ask any trucker; they will tell you those 4-6 inch wide load straps are very spendy.
Right on advice Sharkbait.