@djengan - You might want to try it on a different circuit or even at another location (friend/family house). If it is most predominant just at power-up and then settles down, it might just be normal. Only you can judge if it is excessively loud. As I said, I don't hear a hum from mine, but I can't get to the back side to listen for hum as it's in a rack.
I can tell you, I've had several amps over the years that had a bit of mechanical hum. Once I got a couple of feet away, I couldn't hear it and I did not hear it through the speakers, so I lived with it.
Try it in different circuits and even at different properties to see if it has anything to do with your wiring. Decide from there how to proceed.
Question: How does it sound otherwise? I love mine. Works really well with all of the speakers I've tried, from a couple of different vintage Infinity's to Vandersteen 3's. I also have the matching Rega Saturn-R CD player/transport/DAC and it's a match made in heaven. Really good IMHO.