Olsen's Toy Train Parts has the service manual on line.
Here's the link: http://pictures.olsenstoy.com/library.htm
Unfortunately, I couldn't access the site this morning (not uncommon on weekends). When it's up, enter the library and look for the transformer listings.
IIRC, excessive hum is caused by a loose coil and laminated stack asssembly. The same problem was found with KW transformers. Both units were changed to riveted assemblies (the KW in 1957). Access the KW service section of the site for further information.
Here's a link to a previous topic on transformer humming: http://www.trains.com/trccs/forums/712161/ShowPost.aspx
Unfortunately, the link to the OGR forum topic that I posted in that topic is no longer active. If tightening the ears to compress the laminations doesn't work, you might try posting the question on that forum. The alternative is to live with the humming, or teach it the words.