In bumper to bumper traffic I noticed a minor amount of steam that appeared to be coming from the right front of grill/hood. Earlier there seemed to be a "gurgle" sound coming from the behind the dash. No change in engine temperature and car ran smoothly. Shortly thereafter I lost heat. The next morning I noticed the antifreeze reservoir level was slightly lower. No apparent antifreeze seepage from the radiator or hoses after running engine until warm. Still no heat. Thermostat replaced 3 years/10,000 miles ago. Any prognoses ??