Regardless of what caused the knock, I couldn't re-produce it at the dealer. I even went for a drive with a tech and just couldn't get it to happen.
???
I guess if it doesn't happen that consistently then I shouldn't worry about it. I am under extended warranty for another 40,000 miles, which should take me about 3 years to cover...so if it becomes worse I'll bark more about it. I'm pretty good about getting taken care of...