My experience with high current Class A or Class AB designs with large power transformers like Krell and Classe is that they normally produce a faint mechanical hum when first turned on as transformer charges and “warms up”. The hum should not be audible after warmup. I am not familiar with Hegel in general, but the H400 specs indicate it is a high current design with a large power transformer, so some hum on warm up may be expected. You state this occurs after warm up. This does not appear to be normal. I would contact Hegel for recommendations.
It does not appear you are hearing the hum from your speakers. If you are not hearing it from your speakers I would not expect it to be ground loop hum. If you are hearing it from your speakers, I agree with others to experiment with removing ground loop hum. See the link below that explains ground loop hum and how to attempt to remove it. If that does not solve the problem, then contact Hegel