[majoconz]

I have had a similar problem. I made a unit in an old small speaker box which uses a cheap imported MP3 player, a kitset FM transmitter and a 5v regulator - it all works very well - a 4Gb SD card lays well over 600 oldies - BUT back to the question. When first built there was a lot of hum on the received signal. I was using transformer wall-warts, not switch mode PSUs and was getting to the point where I thought of dumping the whole project - until purely by chance I used a different PSU! Presto - no hum!