It depends what the source of the sound is. If it's indeed the pump motor it might fail or it might be normal for that brand/model. If it's air bubbles it's OK as long as your temps are OK at idle and especially under load. I don't really have a lot of hands-on experience with liquid coolers to know whether it's because of a faulty motor or not.
You can check your CPU temps with applications like GPU-Z or HWINFO. Check the temps at idle and under heavy load and compare them with results online. If they're the same -/+ a few degrees it's OK.