Looks like lubrication was the answer! I had checked and oiled all worm gears and boogie gears but hadn't done the motor bearings where these locos had sat in a box for 3 years not moving due to a house move and new layout construnction! Thanks , all is quiet once again!
That would be somewhat unusual for an Athearn diesel. Apart from lubrication, I would remove the truck covers (from underneath) and inspect the gears. If they seem OK, I would look at the worm gears (on top of the gear tower), after removing the body.
Simon
A very small amount of oil on the motor bearings and the worm gear bearings on each truck.
My AC4400's have both developed an almost grinding sound , sometimes in forward but mostly in reverse. when the sound starts the locomotive slows down slightly! I've removed the shell and placed it lightly on the chassis while on the test track but the noise continues!
Any thoughts!