Lilypad hums and numbers jump around

I bought a USB LilyPad to use in a glove I have made which has a pressure sensor for controlling electronics in electroacoustic music. I find first of all that it emits a high-pitched hum constantly when it is plugged in, which the UNO does not. I also find that while with the UNO, the numbers in the serial monitor remain constant, when I use exactly the same circuit with the LilyPad the numbers jump around (too much to use it for this purpose). Do I have a faulty arduino or is this a known problem with the USB LilyPad? I haven't found any other complaints to suggest that it is...

How is the LilyPad powered? To what is it connected? Are you using analogWrite() for anything?

powered and connected via USB to a macbook pro.

No analogWrite, just read.

emmsok: powered and connected via USB to a macbook pro. No analogWrite, just read.

Very strange. I'd have to know what was connected to the LilyPad and how it was wired to make any further guesses.

The sensor is a strain gauge and the circuit is just a wheatstone bridge. The puzzling thing is that there is no noise and the numbers are stable with the Arduino UNO, but exactly the same circuit attached to the lilypad has these problems.