Cannot modify header - AJAX

[eluser]geogis[/eluser]
Hello,

Having a problem with cookie..

I am using AJAX to run some php script that sets cookie and display a "cart" on the screen. Getting error:

A PHP Error was encountered

Severity: Warning

Message: Cannot modify header information - headers already sent by (output started at /customers/khushimat.pl/khushimat.pl/httpd.www/system/application/controllers/ajax.php:1)

Filename: helpers/cookie_helper.php

Line Number: 92

I tried to find whitespaces and any output. Also saved the file in different editor to make sure there are is no BOM problem. Have updated cookie_helper.php file with a fresh clean copy to make sure there were now changes made. And nothing...

Any help please?