'Funk' noise when iMac waking or going to sleep.

Hi,

My husbands iMac (2011) currently running the newest version of Mountain Lion has started making the 'funk' sound when it goes to sleep or when it wakes up. It's not that big a problem really, but its just getting annoying.

It does it when he manually puts it to sleep as well.
I have tried closing all applications and letting it go to sleep, still does it. Looked through all the settings, couldn't find anything out of the ordinary.

Any ideas?

Did you check System Preferences, Sound, Sound Effects, to see that the "Funk" sound was not selected when the machine gives an alert? Also while you're there, you can turn off all sound effects to make sure.