Tranny brake huh..ya mean a stage rite for the 200..well..that only affects the immediate launch and at 20 mph is something banging somewhere..like motor mount,torque arm..etc..your shifter wont cuase the knock detector to go off.
Plus your one two shift should happen around 40 mph..so..I cant think of anything at 20 unless its something physically banging..
Do you have the knock recorded on direct scan to see if something might be dropping off?
FWIW,last december I had a tranny brake installed and cut a 1.69 60 foot on Nittos at Orlando Speed World..stock pretty much everything..The brake tho was cuasing me to lose 2-3 mph on the top end..very strange..