Gurgling/trapped air in hot water baseboard heaters

I have a forced hot water/oil furnace with two heat zones. The top floor zone has baseboard radiators. When the furnace cycles on, there is a loud gurgling sound and you can hear the water loudly rushing around through the radiators. There dont appear to be any bleeder valves on that loop. My oil company told me to attach a hose to the system, open a valve, flush it out until the water runs cold, then shut the valve back off. This works, but the problem reoccurs in less than a week. I understand that there is trapped air in there but how could it keep happening?? There is no evidence of water leaking out anywhere – I dont understand how air could get into the system.Any advice on how to solve this problem would be appreciated. Thanks!