Alternator

Very possible, if you have a old piece of garden hose around take about two feet and cut it off. Use one end towards ear and the other on the alternator not the part that turns you should be able to hear if it is the alternator by hearing a whirling sound. Check the idler pulley also. Have seen that happen before.

The hose trick I learned from my father, long time ago. Even if the noise stopped you would still be able to hear a difference between a bad bearing a rough sounding noise may change pitch at times and a good one low steady hum.

Sometimes the bearings in pumps and pulleys just wear a bit and end up cutting a new groove (which can make some noise for a while). The alternator on my car starting making that same noise last November. I sprayed the front where the main bolt is and the back where you can see the end of the shaft with WD-40, hoping to just diagnose the problem and get through the work day. Ten months later I have not heard a peep out of it, it is still charging around 14 volts, and just working like it is supposed to.

Maybe that is all that is happening here.