On the chance that it is a loose lamination, find one of these
in your furnace and press on it.
Relays can also buzz but pressing on them is a bit tricky.
With a helper you can also use an empty garden hose as a stethoscope but the metal panels in furnaces can act as diaphragms and so you might do better pressing on things.
The buzz frequency may give a clue; YouTube has various sounds of various frequencies that you can use to compare.