You said "rattle", and that's a good description of the normal noise a fixed gear makes. Sometimes they are silent, when the chain is freshly lubed and the tension is just right, and the planets are aligned and the moon is full, the teeth will mesh perfectly with the chain. But, more likely, there will be a small rattle.

Now, if you said "grinding", or "chirping", that would indicate a problem.

New chains can be noisy at first, then quiet down after a few rides.