Mine had the same - very common - problem. That's the first sign your regulators are failing. Search the site, lots of posts on this.
The RH in my '02 S went last fall; I just ordered the LH, ~$189 + shipping from Porsche dealer Sunset Imports in OR,
http://www.****************************************************.com/ These guys pretty consistently have the best prices on factory parts.
Installation is a fairly easy DIY. Search to find links to step-by-step instructions.