We've been away for a while (touring the French Alps on the tandem) so just saw this thread now.
Last year, we had quite a bit of worrying noise coming from the Gates belt drive after re-assembling the bike after flying with it. I discovered that I had installed the belt in the opposite orientation to what it had been before - flipping it around completely stopped the noise.
The belt now occasionally starts making a noise after riding on wet roads / in the rain. After the water has dried, it makes some noise for a while, but if we keep riding it seems to clean itself off and then is back to being silent again quite soon. I've occasionally tried cleaning the belt with soap, water, and a sponge or brush in these situations, but I was never very successful, and continuing to ride it seems to be the best way to clean it - the open design of the belt-ring seems to work very well.