Joe, I never hear an engine run without sounds just because the battery wasn't installed. The power to run the logic and sounds comes from the power supplies on the bottom of the board. Only time that occurs is if one of the power Supplies on the bottom board is bad. LM337.
This model is from 1997 according to the web, so the battery normally is needed to even run, but I have seen weird issues.
I am going from memory on this, but is there 2 screws on the vertical face of the rear cab. I think they need to be removed, and there may be a screw on the top of the boiler. If not on top, they are under that chassis but higher up near the shell.
Since you removed the motor, once you do get it apart you want to make sure it is aligned properly on reassembly. If the gear lash is off, you will chew up the axle worm gear. G