Andy, these aren't fast runners. Even on the A-U posts it probably won't break 70 scale mph, and that's a good thing!!
As far as the clicking: remember these have no flywheel and a tight worm gear drive. If the loco experiences any kind of momentary voltage drop or dead spot, it will slow down RIGHT AWAY, and can't be "pushed" by the train. So you may be hearing the metal drawbar on the tender slam into the back of the locomotive, or slack bunching in the train. To test this theory, run just the loco by itself and see if you still hear the clicking.
It could also be the pickup rollers snagging in switches or rail gaps. IIRC, the roller diameter on these was pretty small. With a can motor, rubber tires, and worm gear drive, the loco is a quiet runner so all you hear is the "clickety clack."
These were a unique offering, especially when they first came out, and have so much potential. It's a real shame that K-Line and in later years, Lionel couldn't figure out how to install a flywheel!