Ok, sorry to keep you waiting. I had to make chocolate cream puffs and home made chocolate sauce for dinner party tonight. New recipe. I think it's a keeper.
Anyway, about your clunk.... Go out and raise the front bumper, and take a look at that bar that folds up into the spare tire compartment. It's hinged on both sides and might have a fiberglass shield on it to keep the weather and road spray out. There may be a strap attached to it.
I have a feeling you'll find that it's rocking back and forth freely, and when you hit the brakes hard, it rocks forward and smacks the frame where the bumper is attached. Take a piece of rope and tie it back to the tire or frame and give the bus a brake test at 40 mph.
If this is it, you own me a beer. If it's not, I owe you one