I can tell you like precision, and I appreciate that. But in your quest for an exact answer and the resulting consternation...you have not supplied critical info. All you said was in the vicinity of the furnace. That in itself is vague as to location.
Is it during operation?
Is it during down time?
thus my answer which is the ONLY load all the time is 99% true. A transformer resonation is not just the coil vibration or as you say 60 hz...the metal wall, mounts etc dictate the noise which covers the spectrum.
Now...if you came to me with:
The FURNACE is making a noise on START UP...or DURING operation, we can dig further. But with terms like vicinity, no mention of operation or not...I am sure you can see the answer would be difficult.
Plus I think someone else maybe better suited for this question. I will opt out and see if someone can deliver better information on the scenario you have given.
I apologize for your aggravation...After being a Licensed Contractor in 2 states and hired by manufacturers for training their techs I am accustomed to a different reception.
I wish you the best and will opt out so the others can help you. Or if you wish, I will forward to customer service so that the entire amount of $16.00 is refunded.