I recently rewired up a wireless audio bluetooth transmitter to the battery terminals of an old portable player as I always use them together but battery on the speaker is much higher capacity (both 4.2V Li-on). But after doing so, there is a now a background static noise added to the transmitted audio. I did some poking around with the multimeter and the short test seemed to indicate the audio common ground was the same as the battery negative terminal for the transmitter. I'm not much of a signals guy so the only theory I have is that sharing the ground/common (by now being from the same battery) is now giving some kind of ground noise? I've already tried adding a small ceramic cap across power terminals too. I've read 1:1 transformers are a way to solve this as it breaks the wired ground connection, but they don't seem readily available, like a small part I can buy of AliExpress/eBay type of thing. I reckon I'm over-thinking it here; any tips? Anyone had similar issues?