So I was going through the zombie sound folder then I found out that there were 3 unused sound files. Metal 1, 2 and 3. I haven't heard them for a while. I think they were used for hit sound when zombie was wearing iron gear. So I tested it and there were no sound when I hit them with full iron armor. I am pretty sure that they were used in 1.4 or 1.3

(The sound files are not playing properly. Go into your .minecraft folder and play them from there)