Leben CS600 Owners - Help

I have recently become a happy owner of CS600. I will write another post about my overall impression of its sound, but there is one minor issue. There is a very subtle hum coming out of the transformer. It does not interfere with music listening, but I can still hear it from the listening position when the music stops and the room is quiet without AC or other ambient noise. Using the Decibel app on iPhone, I tried to measure the sound level. It is not detectable from the listening position. When I place the iPhone just an inch above the top grill of the amp, the sound level goes up from 55 dB (ambient level) to about 68-9 dB. Should I call it 15 dB increase or just a noise at 69dB?

I am just wondering if this is an expected level of transformer hum for a normal CS600. Can any owner of CS600 try it for yourself and confirm? Much appreciated.