PaulS:
But only when I have the Arduino connected to the USB
When the USB to serial converter actually has something to do.
Meaning that there has to be an application open and running on the PC that has opened the com port, if I'm not mistaken, otherwise the arduino USB converter would not just blindly send serial data to the PC (and blink the xmit led). At least that is my take on the matter and it would be very easy to test that out.
Lefty