Hard disk make click sound

need help- I have done two SPACE defrags and after that two COMPLETE/NAME defrags using O&O Pro V10. The FSX is running well but I hear a click sound from the hard disk and then the flight pause and start again. It looks like loading the files. Now this is happening very often. I have two partitions on the same Hard disk but FSX is installed on C Drive. Other programs are installed on the other partition. Is this because of the partitions or because of O&O defrag setting. Hope Nick give some advice.