I purchased an older Bachmann locomotive with a pancake motor last year at a train show.  However, after I got home and tested it out I heard this strange "clicking" sound.  I ended up turning it into a dummy after I couldn't figure out what was causing the noise. I just thought that it a defect with this locomotive and played it off.

Fast forward to this year.  I recently purchased another older Bachmann locomotive with a pancake motor.  This locomotive had never been run prior to purchase.  Again when I tested it out after receiving it I noticed the same "clicking" sound as the other one.

Was this a problem that plagued the earlier locos or am I just lucky?  And if this was a problem is there a way to rectify it?