Sorry if this is obvious, but you did put oil in the engine, right? After the rebuild?
Coop is right, the hydro is pretty much bullet proof. You only pushed it a short distance, so not likely there is anything wrong with the hydro.
It sounds to me like your hydro is not in neutral while trying to start. It makes an awful squeal when you do this. Move the speed lever back and forth slowly while cranking the engine. At some point in the lever travel, you'll lose the squeal and the hydro will be in neutral. You may need to bypass the brake pedal safety switch. When the brake pedal is down, and the neutral is not properly adjusted, the tractor will try to move forward or backward while cranking, and will make a lot of noise.
I'm betting once you get it started, find neutral with the speed lever, the noise will go away.
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