Studio 1557 occasionally cannot boot up

Hi all, I have a studio 1557 running on windows 7 which recently has had the problem of not being able to boot up properly. The screen freezes at a certain point in the boot up process. For example, it might stop at the screen that shows the dell logo and not continue booting up. I do heard some rather high pitched whine coming from the machine. Once or twice I got the message "OS not found". I'm wondering if something is loose, such as the HDD connection.

Can anyone help? Thanks.