The egg was darker than others and we didn't candle it because we were too worried we might damage the egg further.
We picked up our cockatiels egg and we were about to candle it but we felt something hart rattling inside is it alive?
1Welcome at pets.SE! As much as I know, candeling eggs does not damage them.– AllerleirauhFeb 21 at 15:34
Candling an egg will not damage it unless you are using a literal candle and hold it too close. That being said, I'd say you should not use a candle and instead try a flashlight or something that does not involve fire. I do not raise birds, so I don't know much about them, but I did do some research and found a wikihow article about how to do it. There was not anything on egg damage, so I'd assume damage is not a risk.
As for the hard thing inside the egg, I cannot answer that, because it could be a lot of things.