Update: SOLVED! Working with the Service Advisor, we learned that the vehicle was behind on software updates. I choked at the price, as it was over $600. SA claimed that Volvo actually charges the dealer for these, so he could not mark them down. (We also learned that other major car manufacturers, like Audi, also charge their customers in this manner.)
In attempting to initiate the downloads, it was discovered that the USB port was bad. Another $128.
Bottom line: High-Pitched whistle is gone! Not sure which fix solved this problem.