Correct me if I am wrong, but as I understand you need to display a loading panel during an AJAX call to bind RadGrid that is located inside RadSlidingPane. To achieve this I would recommend you to use RadAjaxManager to ajaxify the sliding pane's content. however, since it is not possble to direcly ajaxify the sliding pane, you need to use another container control to wrap the content and set this control as updated control.
For your convenience I have attached a sample page demonstrating the above mentioned approach.
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