What you are hearing is commonly refer to "filament sing". The way the semiconductor dimmers work is by turning on midway into the wave form. It should be worst at about half brightness (because that is biggest excursion, full off or full on isn't any different than a regular switch).

Why you never noticed it before is likely that the bulb is different or it's mount in the fixture makes the problem worse. If we're talking regular bulbs, you can try a "rough service" bulb to see if it has less of an issue.

What kind of light fixture?

Usually a different brand of bulb will solve the problem.

Thanks everyone...It is a semi-flush mount in my kitchen. I will try different bulbs. I really just wanted to make sure I had not made a mistake with the installation. You have all been very helpful....

Try rough service or ceiling fan bulbs. They have filaments with stabilizing supports and should vibrate less.