Speakers low pitch humming, is it normal, or real issue with preamp?

I shall appreciate your experienced inputs. I just purchased pre-owned audio set, all used for 2 years and in good conditions. McIntosh Preamp C53, Amp MA462, B&W 805 d5. I noticed whenever the preamp was turned on, there is a very low humming noise coming out from speakers, can hear it only when get my ear very close to the speakers ( so probably it does not affect my normal music listening which mostly covers the noise). When I tried turned off preamp, and keep the power amp on, there is absolutely no noise, so the noise definitely comes from the C53. When I connect to the DA2 using digital input with coax cable, the noise is more pronounced (even without streaming source). I am a newbie for audio, am I too nervous, or this is normal since no component is perfect?