Sounds like you didn't get it into gear all the way, let the clutch out, and ground the syncros a bit. What you heard is basically the syncronizer teeth grinding together because they had not been given enough time or force from the gear lever to mesh completely before the clutch was released.
This is certainly not the recipe for longevity of a tranny, but they can take a few gear crunches at low speeds with no real permanent damage.
Don't worry about it, just don't do it very often.
The classic symptom of worn/damaged syncros is notchy shift feel, kickback on the lever, difficulty going into gear, or more grinding, even with a good shift motion. if it still shifts fine, forget it.

Oh, one of the toughest things on the syncros is a downshift to 1st while just coasting down at, say 20MPH. The low engine speed and lower gear ratio of 1st gear makes the syncros work hard to match the gear and the engine speed.

DG