Front bumper question for those with Viper cuts or C4 Lo Pro Bumpers w/ High Clearance additions.

I recently had a local 4x4 shop install a C4 Lo Pro Stealth bumper with the high clearance additions. As part of the installation they cut off the bottom 8 to 10 inches of the wheel well liners which I figured was fine. However what I have noticed is if I go over 80 on the highway the air starts causing the bumper to cavitate and make a banging noise. I looked it over and it was fine but I'm wondering if I should get some heat moldable plastic and fabricate some panels to seal it better from the wind? My other thought is to get behind the part of the bumper that now flaps and support it with something. I'm curious what others have done in this situation? The benefit at least is I drove home from Denver over the weekend and kept it at 75MPH saving me a little gas! LOL