As you know, I had a "Thrum" at speeds above 50 mph. I found that it was the tires (Falken FK452) that just didn't like some road surfaces.

The rim/tires balanced just fine. If you can get it up on a lift, (in neutral) spin each wheel by hand and look for any wobble or out-of-round either in the wheel or tire. I have a good eye for such things, but some may have to hold a tool (non-scratching) up against them to see any differences. Most tire shops have guys that really pay no attention to what they are doing-just opening ones eyes helps a great deal!

Even after I took mine to a "performance shop" the manager told me that short wheelbase cars like ours are more prone to be finicky. IDK