I bought an old American Flyer No. 2 transformer (60 Cycles - 75 Watts) at a train show recently. I'm planning on using it just to light up a few of the lighted houses and buildings in my kids' layout. When I plug it in, it has a hum to it. When I hook it up to some of the lighted houses, it hums a bit louder as I crank up the power. Otherwise, it seems to work just fine. Is this normal for an old steel transformer?