Just as a test, try lifting the ground plug on your preamp power cord. Use one of those cheater plugs that will allow you to use a two prong socket. If the noise goes away, you are probably looking at a ground loop. If it does eliminate your problem, "don't continue using it this way. It is not safe but it will tell you if you have a ground loop problem.
On another note, most transformers allow for 110 and 220v connections. Did you ask the manufacturer? If not you could have the transformer changed to a 110v transformer. Step-up and step-down transformers tend to cause some issues.
On another note, most transformers allow for 110 and 220v connections. Did you ask the manufacturer? If not you could have the transformer changed to a 110v transformer. Step-up and step-down transformers tend to cause some issues.