Are you certain that this is being caused by nCrunch.BuildHost? I would expect that only the engine process should be interacting with the hash cache.
The hash cache does, unfortunately, need to be stored somewhere persistently. This cache is used to allow the node to quickly identify matching snapshots for connecting clients, and it does contain relevant data that can be quite expensive to reconstruct.
Do you have any information about whether the interactions with the hash cache are read or write? Also, which files under Local\Temp are being used? If this is being done by the buildhost, it may well be MSBuild tasks doing this.
