This might not help with your problem, but it is a story that I think is worth sharing.
I am flying to London on Monday, and I went out earlier this week to buy a converter. At one general purpose electronics store, the sales assistant (under 20 years old, I think) had absolutely no idea what he was talking about. There are two types of these converters, he said. That proved to be true. One is for heating appliances (e.g., hair dryers), and the other for computers or TVs, or similar electronic devices. That also proved to be true. But then he said that if you plug a computer into the one designed for hair dryers, it would destroy the computer. His reason was that the volts would be OK for the computer, but the amps would be too high. I was incredulous. I started to explain what volts and amps were, but I quickly saw the ?deer in the headlights? look, and gave up.
Then I noticed that the package for the heating appliance type of converter did say that it should not be used for computers. That really threw me. I could not imagine how a computer, which draws far less power than a hair dryer, could be damaged by a device that has a higher power rating than the computer needs.
I wound up buying a transformer (240-120) from another store. The sales assistant there gave me a reasonable explanation for the anomaly I describe above. He said that the converter devices intended for heating appliances are rated for short term usage only. You generally need but a few minutes to dry hair. But you would leave a TV running for hours, and you might leave a computer plugged in and recharging overnight. That would cause an overheating failure of the converter, and would then place the electronics at risk.
My guess about your buzzing problem is that you are drawing more current than the converter can handle. That is causing a large voltage drop, and buzz comes from the motor's reaction to the low voltage.