Sounds like the sound effect for the sheep is clipping badly (that clicking sound) - usually a result of a bugged audio file or the program lagging in playing the file (from what I understand sound effects in Trainz are uncompressed WAV files so they should theoretically load immediately, but if you have lots of that same asset (in this case, a sheep) all close by, they're all trying to play their sound effect at the same time, which naturally can cause things to start to lag, thus the clicking sound (assuming those sheeps are lots of individual assets and not just a single group asset).
It's like if you try to open too many windows/tabs in Windows - one or two isn't going to be a problem but ten or more and things tart to slow down, and the more you add the more overloaded things get. Instead of using individual sheep assets close together, try looking for animal groups which should only play the one file and hopefully not have that audio clipping issue.