Hi jcm2016,
I initially assumed the problem was signal distortion. Obviously, in the present case it is "only" about a buzz/hum that can be heard in addition to the (otherwise clean) signal. Presumably it is a so-called "mains hum", which can occur for various reasons.
To test, please ensure that no other cables except the external power adapter are connected to the SX900. For example, a so-called "hum loop" could occur through a USB cable that is used to connect the keyboard to a computer.
Also make sure that the external power adapter is not on the keyboard or very close to the keyboard. Under no circumstances should the power adapter be near cables that are connected to the keyboard.
Try turning the plug of the power adapter in the wall socket by 180 degrees. Sometimes this helps in such cases.
Try different sockets in the house (using a mains extension cord if necessary).
Under certain circumstances the power adapter could be defective (so that the 50/60Hz mains hum comes into the 16V DC output voltage).
If you have already tried everything mentioned here, I recommend giving the keyboard a try at a friend's house or at your music store. If this doesn't work either, you could try using a different power adapter (e.g. on loan from your music store) to make sure that this is not the problem.
Hope this helps!
Best regards,
Chris