Are there any constant noises near the racks? Do you feed strictly 4018? It's only 5% fat, which I think is kinda low. When I had rats I fed them this block but I would supplement their diet with sunflower seeds and the occasional few pieces of cat kibble to up the fat in their diet.
What other problems have you had with the rats? Are your rats tame/handleable or skittish, and do you handle them a lot?
never noisy where they are, and i do feel 4018 but i also put peanut butter in their tub sometimes to give them a boost. Some of the rats are tame but i don't handle them a lot since i want to give them their peace.