Diagnosis
That's thermal expansion and contraction. The ceramic heating element and its metal housing expand when they heat up and contract when they cool, and they can make popping or ticking sounds as they move against each other or the plastic frame.
How to Fix It
This is generally normal and not a sign of a defect. There's no repair to stop it entirely, as it's a physical property of the materials. You can try to minimize it by making sure all the screws on the exterior casing are snug (but don't over-tighten plastic). Also, ensure the heater is sitting firmly on all four feet on a level surface to reduce any slight rocking that might amplify the sound.
If That Didn't Work
If the clicking is constant, rhythmic, or sounds more like a grinding or scraping, it's a different problem. Unplug the unit and gently rotate the fan blade through the grille to see if it's hitting something. A foreign object inside or a bent fan blade can cause repetitive noise.
Heads Up
A single, loud *POP* or *BANG* when turning on, followed by a burning smell and no heat, is a serious sign. It indicates the ceramic element may have cracked or shorted. Unplug it immediately and do not use it again. That's a failure, not normal expansion noise.