MH Ballast help

[mike31154]

I gather this is a magnetic ballast? Transformer/coil & capacitor. To produce 400watts takes some large components and transformer hum can be substantial. Have you opened the unit up to see what's making the noise? Could be as simple as a loose mounting screw holding the transformer in place. Or you might look into replacing the capacitor if it's that old. If saving money is the goal I'd say it's worth having a look at these things. It's still firing your lamp(s) so there's nothing to lose by checking for loose connections.