The http headers was the clue that lead to my solution. The chunk was being compressed by Gzip, so I used the Zlib library to unzip the chunk in order to get at the xml inside.
var zlib = require('zlib');
// Handle proxy response data
proxyResponse.on (
'data',
function ( chunk ){
if ( debugging ){
console.log ( ' < chunk = %d bytes', chunk.length );
}
xml += chunk;
response.write ( chunk );
zlib.unzip(chunk, function(err, chunk){
if (!err){
console.log('Response'+chunk.toString())
}
});
});