Create your Rate Column as a Template. Inside the template put a control (anyone that you need. E.g Rating control from AJAX Toolkit)
So assign the title and/or tooltip property to:
title='<%# Bind("Comments_Column_Name")>' tooltip='<%# Bind("Comments_column_name")>'
Comments_Column_Name is the name of your column as you explain on your question =)
Hope you can find it useful.