Ive unwrapped transformers with hissing or humming noise. When unwrapped, usually you will notice a light blue light emitting from the transformer components. This is caused when the copper coils are too close or have lost their shield coating. If this is your problem, you should consider replacing the item. This happened to my dewalt cordless fluorescent area light. CORDLESS!

When I unwrapped the transformer, reconnected the power supply and found the strands that were bridging, I disconnected the pwr supply, took a tiny flat and slowly separated the strands until the problem was resolved. This is extremely unsafe if you are not trained, I was fortunate that this wasn't a household appliance. Overall, just replace the item. If its a low voltage and current (battery powered device), work on it at your on risk. It is unwise to work with loaded circuits. I only mention my findings, so you understand a common cause, and not intended for instructional purpose.

If the problem hissing is from the transformer than this may be caused when the copper coils are too close or have lost their shield coating. If this is your problem, you should consider replacing the item because it is extremely unsafe. Overall, just replace the item. It is unwise to work with loaded circuits.