PC constantly reads diskette drive

This is a very strange PC problem I have heard before from some users. Every now and then, the machine tries to access something in the diskette drive, even if the drive is not being used by any program. This can be quite an annoyance, especially atnight. I have tried all the easy stuff (check the BIOS and antivirus settings, etc). Can anybody help me with this one? Thanks in advance and regards.