Dac acts like a microphone

I replaced the stock tudes in my Audio Note Dac to used Amperex 6922 tubes. When I powered the unit up there was a strange sound (as the sound of an AC unit running) coming from the left speaker. I took the tubes out and put them back in. The dac was now picking up sounds from my tapping the dac through the speakers. I got the feeling that I could sing into the dac and it would act like a microphone. As the tubes got hot, and two hours of running, I had no problems. Will this happen again because the tubes are bad?.

Thanks,
Gary