I have an old athearn gp9 from about 1975. I pulled it out after about 30 years and ran it. It ran slow and would stop completely after about 4 rounds on a 4 X 6 layout. It ran very quiet. I replaced the motor and now it runs great but there is a loud noise ( seems like it is in the front trucks) when running forward. Seems like it make more noise when on a curve. When it runs backwards, there is still a noise but it is not as loud as going forward. The trucks and gears have been oiled and greased. What could be the problem - cracked gear may be? I don't know what to do. I though I should try changing the front truck assembly but I haven't found one yet. I don't know what but something happened with the remotor. I like this engine and would want to keep using it. Anyone have any ideas?

Perhaps the universal joint?

How can I tell if it is the u joint and how do I fix it? Replace it of grease it?

I would look to see if it is cracked or misaligned.

One thing I forgot to mention. The backward & forward movement of the engine was just the opposite of all my other engines and the noise was coming from the rear trucks. So I moved the trucks front to back and back to front and now the front is making the noise.

I think that you should open the noisy truck and check all toothwheels for correctness, cracks, broken tooths.
Second mount cleaned truck parts together without the worm - and it should run free with a very light fingertip.
however I think that one of the toothwheels should have a heavy demage. Than you should replace it or the whole truck.

Thanks for the info. I will take the truck apart and see what I can find.

I took the front trucks off the GP9, took them apart and I could not see any broken teeth or chips. I had another truck from I think a life like locomotive. I took that apart and the gears looked the same size when I placed them together. I than swapped the gears from the life like to my athearn and they were a match. I put the athearn back together, greased it up an put it on the tracks. I still have a slight hum but not anything close to what it was before. It must be the athearm hum that I always hear about. I will let it run for a while to break in the new gears and will report back. Thanks for the info given me. I really appreciate all the help as I have been out of model railroading for more than 30 years and pulled my trains out of the attic about 6 months ago. I know I have missed a lot of needed knowledge in those years and hope to catch up. Also, I am a little clumsy with my hands and I also have some arthritis in them but I am determined to learn as much as I can and fix what I can.

A next idea.
Do sit very fine washers left and right on the worm shaft avoiding rubbing against bearings?
The worm moves with turning to the bearings - in both directions dependent from rotation direction of worm/motor.
First the worm must have a little bit movability an axial direction but it must be limited by washers. And second the worm can be pressed to one of end if the following gear-wheels are turning hard. That all can generate a heavy rolling loco or truck and in all cases this will be generate also "noise". I hope for success.