XElement Save Adds Leading Charachters

Dear Experts,

XElement objEl = XElement.Load(Filename);
            objEl = objEl.Element("ACES");
            XNamespace xsi = XNamespace.Get(@"http://www.w3.org/2001/XMLSchema-instance");
            XNamespace aces = XNamespace.Get(@"ACES_DLR.xsd");
            objEl.SetAttributeValue(XNamespace.Xmlns + "xsi", xsi);
            objEl.Add(new XAttribute(xsi + "noNamespaceSchemaLocation", aces));
            objEl.Save(Filename);

after the file is saved the saved file have these 3 characters ∩╗┐ , ascii vales[239,187,191]  how to suppress this 3 charachteres while write and   can you explain why they get added and what is there purpose