A couple of dilettente ideas:
- Is a cable-end on either derallieur touching the chain?
- could you have a loose reflective light or valve screw on a wheel?
- could your bracket be worn out/second hand?
- is it possible that your front derallieur chain ring thingo might be a bit high or low, causing the chain to drag on the top or bottom of it? that you've only been checking for a lateral touch?
considering that it's such a consistent thing, it seems obvious enough that the chain must be causing it. 'dsaul' seems like he's onto something...