I have an I2C FM transmitter for the pi; it has a 3.5mm stereo jack for input.
Everything works great -- I attached a short dipole antennae and get clear reception for at least 25'. However, if I use the same model B pi to provide the input, there's a slight steady noise; sometimes this rises and falls in pitch/intensity. I've heard it do this at precise one second intervals, presumably synced with another device on the same I2C bus that receives updates at that rate. However, usually there's no such correspondence. The noise is there whether or not the pi is actually playing sound, although it is not very noticeable if it is (i.e. the relative volume is very low).
If I use two pis, one powering the transmitter and one providing the input (or just use some other device for input), it's crystal clear. I've tried various power supplies, which did not make any difference.
Is there anything I can do to filter or correct this? What's it caused by?