Hi William, I am not an engineer, but I think that you have done all the things I would do to find the knock. I would be inclined to take off he wheels and examine them carefully prior to anything else. I had a knock at certain speeds some years ago, and found some damage to a tyre on the inside. And while the wheels are off, take a good look at the suspension and shock absorbers. Bad roads and potholes do take a toll on those bits!! Regards, Peter.