This is pretty widely-known, but I haven't seen a topic about it, so I figure I'll post here:
If you have problems playing Crash Course due to a strange noise looping and overwhelming the rest of the in-game sounds, then it's most likely the result of Crash Course files mingling with files they aren't supposed to. Custom campaigns that are known to cause this are:
- Back to School (Beta 6.0)
- Silent Hill 1 (Beta 5.0)
In order to get rid of the sound, go to the main menu, select Extras, then Add-Ons, and disable the campaigns that cause the issue. You can also uninstall them to achieve the same effect.
Back to School is also known to cause the main menu background to disappear after playing it. Disabling it in the add-ons menu will restore the background.