Writing an XML fragment using XmlWriterSettings and XmlSerializer is giving an extra character

I need to write an XML fragment to be consumed by a web service. Any xml declarations cause the web service to reject the request. To support this I have the following class:

public class ContentQueryCriteria
{
    public int Type { get; set; }
    public string Value { get; set; }
    public int Condition { get; set; }
}

which allow me to set the request criteria and then get the results.

The code is used like this:

ContentQueryCriteria content = new ContentQueryCriteria();
            content.Type = 1;
            content.Value = "NAVS500";
            content.Condition = 1;

            string requestBody = SerializeToString(content);
            Console.WriteLine(requestBody);

When I serialize this to an XML file I get a proper response, without the XML declaration or any namespaces. However, I would rather capture the data in a memory stream rather then a file.

Using the following method (taken from http://www.codeproject.com/Articles/58287/XML-Serialization-Tips-Tricks ) I am able to achieve results, but for some reason I have a ? listed as part of the string.

public static string SerializeToString(object obj)
{
    XmlSerializer serializer = new XmlSerializer(obj.GetType());
    XmlSerializerNamespaces ns = new XmlSerializerNamespaces();
    ns.Add("", "");

    MemoryStream ms = new MemoryStream();

    XmlWriterSettings settings = new XmlWriterSettings();
    settings.OmitXmlDeclaration = true;
    settings.Encoding = Encoding.Unicode;
    XmlWriter writer = XmlWriter.Create(ms, settings);
    serializer.Serialize(writer, obj, ns);

    return Encoding.Unicode.GetString(ms.ToArray());

} 

the resulting string is:

?<ContentQueryCriteria><Type>1</Type><Value>NAVS500</Value><Condition>1</Condition></ContentQueryCriteria>

if I set OmitXmlDeclaration = false I get the following string:

?<?xml version="1.0" encoding="utf-16"?><ContentQueryCriteria><Type>1</Type><Value>NAVS500</Value><Condition>1</Condition></ContentQueryCriteria>

Can anyone help me determine why the extra ? is there and how I can remove it?

Working SerializeToString method with no BOM

public static string SerializeToString(object obj)
{
    XmlSerializer serializer = new XmlSerializer(obj.GetType());
    XmlSerializerNamespaces ns = new XmlSerializerNamespaces();
    ns.Add("", "");
    MemoryStream ms = new MemoryStream();
    XmlWriterSettings settings = new XmlWriterSettings();
    settings.OmitXmlDeclaration = true;
    settings.Encoding = new UnicodeEncoding(bigEndian: false, byteOrderMark: false);
    XmlWriter writer = XmlWriter.Create(ms, settings);
    serializer.Serialize(writer, obj, ns);

    return Encoding.Unicode.GetString(ms.ToArray());


}