I isolated the problem to the belt idler wheel by spraying lubricant on different components until the noise stopped. I also noticed the belt was starting to crack. I removed the idler wheel arm and cleaned it up and put a dab of grease on the wheel shaft. I then re-installed the arm, and the problem was solved, at least short term. I then ordered a replacement idler wheel and a belt, anticipating that I will need to replace these items sometime in the relatively near future.