Just closing this one out...
Sent it to MTH. Got it back in a week!
Unfortunately, it did not misbehave when it was there, operating "normally" and was returned as was. So of course, when I tried to run it at home, the sound system went haywire. Shot a video and sent it to MTH Service. They directed me to return it, and would be addressed as a priority repair.
3 Weeks later, it was returned. They replaced the "AE-1000030 Tender Board". All seems to be operating normally.
Kirstin at MTH Customer service could not have been nicer!
All in all, a good experience with MTH, and I am happy with the locomotive.