Quote Originally Posted by Dork_Forge View Post
I'd imagine the issue would be touching the illusion more than the noise.
Both. If you observe the suspect, you'll notice any 'clipping issues', you'll notice if the sound doesn't come at the right time, and you'll notice if someone's casting Minor Illusion to fake the sound. You can also tell them to hit the shield in specific pattern, so MI sound can't be prepared in advance.