As discussed in this thread on www.TheRailwire.net : http://www.therailwire.net/forum/index.php?topic=40758.30

Ron Bearden shared with us a technique for minimizing noise caused by worm gear 'rattle' on long wheelbase
N scale split frame, universal joint drivetrain diesel locomotive mechanisms.

The thread describing this modification is: http://forum.atlasrr.com/forum/topic.asp?ARCHIVE=true&TOPIC_ID=43690 - Ron Bearden's post of 2008 January 13 : 3:23:31 PM

This photo shows how, as described in the thread above, Ron hit on the idea of cutting a Rapido coupler spring
in half, and inserting it into the flywheel or the universal joint. The effect is to put a SLIGHT amount of
outward pressure on the worm shaft and worm gear... thus reducing the amount of "worm gear rattle", and thus
reducing the noise made by an offending N scale split frame, universal joint drivetrain diesel locomotive mechanism.

This shows the right side.... we did the same on the left side. See YouTube showing the resulting improvement here:
https://www.youtube.com/watch?v=XtsUkADRR98 and here: https://www.youtube.com/watch?v=0lXDr3A3X0s