Consider yourself lucky and leave it alone! Disabling the ASD is one of the first things I plan on doing next weekend once I get my car. If I wanted fake engine noise, I would have ordered a fake engine!
If you really want it and have never coded it out, it's probably safe to assume it's not coded out. Especially since it's intermittent, the place to start would be wiring as BentZero suggested. Check the connector at the amp in the trunk. You'll need to remove the trunk liner to find it. Look for instructions on how to disable ASD; you'll find several videos on YouTube to guide you to it. If that's not it, take it back to the dealer. I would think it would be covered under warranty if something is loose.