All,
Thanks for the responses.
I replaced the thermostat and radiator cap just over a year ago. My recollection is that the gurgling started in the summer and the leak (coolant level low condition) began in late August. So, I really can't say that this began when I changed the thermostat. That was the first place I looked for the leak, but none is apparent there.
Check out this other thread too.
It seems to have info relevant to coolant leak problems.