DIAGNOSIS This is normal thermal expansion noise, not a leak. The sound is oil and air inside the sealed columns expanding as they heat up.
FIX No action is required. The sounds (gurgling, light ticking) are typical during the initial heat-up phase and will diminish as the unit reaches operating temperature. Do not attempt to drain or add oil.
IF THAT DIDN'T WORK If the sound is a loud, continuous knocking, the heater may be on an uneven floor. Ensure all four feet are firmly on a level surface.
HEADS UP The unit is permanently sealed. Any visible oil residue on the outside indicates a serious internal fault—unplug immediately and contact service.