If you wish to limit your search to a specific folder you can add something like this:
var TopSearchFolderID = new Guid("{A5D56753-E5E4-42A1-AD3E-4B43C70CC618}");
var result = query.Where(x => x.Content.Contains(_searchTerm) & x._path == TopSearchFolderID.ToString("N"));
If you wish to exclude specific folders you can do something like this:
var FolderToExcludeID = new Guid("{A5D56753-E5E4-42A1-AD3E-4B43C70CC618}");
var result = query.Where(x => x.Content.Contains(_searchTerm) & x._path != FolderToExcludeID.ToString("N"));