questions about a self-made mini chat

Hi there, i have some questions to solve some problems, i recently encountered with my self made mini chat script. Ok, i use the meta-refresh in the main chat-messages frame to update the frame every few seconds. In windows OS, you always get that "click" sounds whenever it refreshes the frame. Is there any way to stop this? I provided the script with a "Users online"-list, but when a user simply closes the browser, he won't get deleted from the list, because of not having used the "logout" function. What would be a nice way to catch this exception? I thought about adding a timestamp to the user online list table and every time the user submits s.th. new the timestamp gets updated. If the user didnt post for about ...lets say 5 minutes, then he gets deleted from the table by the script. ...well, thats just what i thought about so far, but would this cause too much MySQL traffic? Regards, Duncan