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Most high quality audio systems (which include an FM tuner) provide an antenna input (either 75 or 300 Ohms. The most convenient antenna input is usually the 75 Ohm Coaxial cable rf input to the receiver. While you may have lifted all of the grounds on your system equipment, it is likely that you did not provide an isolation balance on the rf input presenting you with a continuous and low level hum sound. This problem can be solved with a balun transformer having isolation between it's input and its output. What a wonderful sound when that ground loop is broken, you will then realize just how loud that hum has been and you will be glad to get rid of it.
Craig
ps - note that this noise will get into your audio system even when you are not using the tuner but playing some other input like the CD player.
Craig
ps - note that this noise will get into your audio system even when you are not using the tuner but playing some other input like the CD player.