MacBook hard disk never stops

My MacBook (Late 2008, 2 GHz Intel Core 2 Duo) running OS X Lion (10.7.5) has a hard disk that will not stop running lately, even during sleep. I have a Time Capsule using Time Machine and have iCloud running with iPhone 5 and 2 iPads in the house. However, this has never been a problem before. When I have the MacBook running on battery in the house, the batteries even run down in less than 2 hours due to the hard disk and fan constantly running. Also, it seems to be affecting the speed of the wifi internet access and of the Time Machine backups. Any ideas please?