Diagnosis
Totally normal, and it freaks everyone out the first time. The metal fins and the oil inside are expanding as they heat up and contracting as they cool down, which causes ticking and popping sounds.
How to Fix It
There's no fix because it's not broken. It's just physics. The noises will be most pronounced when the heater first reaches its target temperature and the thermostat cycles the electric element off. You'll hear a series of ticks and pops as things start to cool and contract.
If That Didn't Work
If the noise is a loud, persistent buzzing or humming (not ticking), that could be the fan motor or a loose component. Unplug the unit and check that the plastic front grille and rear cover are snapped securely into place, as vibrations can cause buzzing.
Heads Up
The noises are a sign it's working as designed. Don't mistake this for the sound of leaking oil—that's not possible unless the sealed metal fins are physically punctured, which would be obvious.