Context.RewritePath

Hi i am using Context.RewritePath to rewrite url aliases which i have stored in database (code below), this is working fine apart from when i do a Aysnc postback then this happens
when the page is loaded the URL reads http://sitename.com/contactus/
with Default.aspx?id=2 written behind it
after aysnc is fired the URL is http://sitename.com/contactus/Default.aspx?id=2
any ideas how i can stop the Default.aspx?id=2 being displayed?
 
**CODE**
void Application_BeginRequest(object sender, EventArgs e)
{
                var alias = data.UrlAlias_Select_Url(Request.Url.AbsolutePath); //check database to get the ULR and id for the page
                if (alias != null)
                {< ...