Hi, Kevin & everyone,
Drumming (I believe this is the korrect speling *S*), is pretty much as Kevin described, and IIRC, this can happen in any steam locomotive.
A long time ago there was an article (in TRAINS?) about steam engines, and the drumming phenomenon was mentioned, including that the 844 sometimes experienced drumming. The cure (again, this is from memory) was to change the fuel/air ratio or the throttle setting.
Phil, Rick, or the other experienced steam people can confirm or correct what I've written.
-Jim