The volume on my Imac keeps going up to full volume of its own accord and staying there.

The volume on my iMac keeps going up to full volume of its own accord and staying there. If I turn it down manually, it does the same thing again seconds later. It’s a PowerPC iMac running 10.5.8 with no external peripherals or speakers. I’ve turned off the 'play feedback when volume is changed' in system preferences, so the sound alert has stopped, but the volume speaker overlay is still showing on screen with the volume set to full. This started randomly about a week ago, there have been no recent software updates (it's my secondary mac) so I've no idea what has caused it. I've tried an old keyboard and the fault still happens. I've re-installed 10.5.8 too. Out of ideas now.