the gearbox(es) in the trucks (might have only one, might have two). If the gears are visible, you can just put a drop or two of oil on the exposed bits, and run the loco forward and backward ( holding it upside down in your hand, so you don’t drip oil all over the place). If they’re not visible, you’re going to need to get the shell open to oil them (read below).
the motor shaft/bearings. For this you’ll need to get the shell off the loco… I’m no expert in the makings of AHM diesels (or any for that matter), but it will either pop off by gently prying the sides apart from the chassis, or there will be a screw or two in the fuel tank holding it together.
I only have steamers (and none too many at that), so I can’t suggest anything beyond labelle oil for the gears/bearings/etc.
One other thing that mightbe causing noise is excessive play between the gears in the gearbox, so you should probably check that too, although I wouldn’t know how to go about fixing that problem…