I had an intermittent hum that would stop when I wiggled the RCA connector on the back. I found a bad solder joint inside on the PCB that has the output jacks. It went away for a while but now it's back. Just tapping the case makes it go away. I need to go looking inside for another bad connection. The microphonics (mechanical reproduction) are very likely caused by the same problem. If you are not a technician yourself, it will probably cost a bit to have one open the unit and look around. The quality control on the subwoofer output board must not have been very good.
If you do take it to somebody, use a shop that sells audio equipment or at least does the warranty work on audio gear. I bought an earlier model really cheap on Ebay and found that the mica insulators under the output transistors had been removed. The overload circuitry kept that from burning out the transistors. I replaced the one bad transistor and all the insulators and ended up reselling the unit at a profit. I was going to use it with unpowered subwoofers but changed my mind.
60 cycle hum is almost always a grounding problem. Plug an RCA cord into an input and touch your finger to the center conductor on the other end and you will get a 60 cycle hum. (it will be in the regular speakers too) (no, I'm not trying to get you shocked)
On SonyStyle.com go to "Support", "Product Support", "replacement Parts", go to the bottom and click on "click here" for the parts research form, under "Detailed Question" click on "click her for details" and if you live near one of the distributors listed, I would go to them. If not, I would call the nearest one and ask if they know someone in your area.