I've experienced three types of buzzing sounds.
A bad powersupply caused sound issues on an A lens camera. But I have wondered if it could be possible that it was the power supply and card combination causing the problem, as it was an unknown brand card that I borrowed with the camera.
Rattling buttons on my C lens, that sounded like buzzing at certain engine rpms. This was fixed by bending the buttons and adding a dab of silicone.
Rattling battery cable at certain rpm, when I didn't tuck the cable in quite right. This was temporarily fixed by putting it back in the same way the factory had it. Once my lens issues are sorted I'll be putting in the super cap and using some self adhesive foam.