It's it common for transformers to buzz? I hear it when the unit isn't running think it always does it on or off.

It's it common for transformers to buzz? I hear it when the unit isn't running think it always does it on or off.

Transformers are always powered on so the low voltage control system can send and receive signals to operate the system itself. They will make noise since most of them are from China now and the metal plates that help make the core of the transformer shell are not manufactured with tight enough specifications.