No need to worry. It's perfectly normal for your oil filled heater to do that. It's just the cooling and heating of it.
The metal is expanding and contracting causing the popping and cracking noises.
The same can happen with your house.
If you heat up the house you might hear creaking noises overnight or during the day as the house heats up and cools down.
Perfectly normal too.
I have one and it does the same thing.