Identifying Noise in 12AU7 Based Headphone Amplifier
After looking at several different forms of transistor and general circuit noise, I accidentally discovered the answer was RF interference from my cell phone.  I'm not sure which specific part signal it is, but I notice it more when my phone is sending and receiving text messages.  My carrier is Verizon and phone Samsung, I don't have access to any others to determine if it is specific to this or a problem in general.
This noise is not unique to my amp.  I've since heard the noise in several solid-state amplifiers including a turntable with built-in equalizer amp and a digital piano with built-in speakers.  I'm assuming the RF interference could be mitigated with some better design, but that will have to wait for another day.