No, likely not a fault. Every transformer will react slightly differently to whatever it is on the mains that is causing the hum; some will be quiet (but may be noisy with different artefacts on a different supply), some will hum. On a perfectly clean supply, with no DC or other artefacts, you would expect no hum from any toroidal transformer.
As for Naim, the transformers are tested by the maker (currently Nuvotem) and then again by Naim at the factory. Naim have a device that will replicate many different types of noise that may be found on a mains supply. Any transformer that exceeds a set limit is rejected. Of course, the test cannot replicate everything. Certain devices can dump DC locally onto the mains. Just try running a hairdryer or anything that uses a simple heated element in the house and see what happens. Also things like fridges can be notorious culprits if they’re not in good health.