pro tools generates wierd hum

[idanaltman]

when I have more than 20 channels open, pro tools will generate a strange constant humming sound. when I bounce a track inside PT this hum is not added to the audio but when I mix externally (which is the only way I work) this hum is amplified and of course added to the track.
I can hear this hum even when I'm plugged directly to my interface's headphones out but it is significantly louder when I send the mix through my mixer.
I do not use any plugins but the pro tools compressor every now and then (I don't mix ITB). this hum is only generated when I load pro tools after I open a session.
does anyone know where this is coming from?

[albee1952]

Could be lots of things but you give zero details about your system. Anything from hum to a cheapo USB cable, a ground loop with your monitors, bad laptop brick...............The fact that it doesn't get recorded, points to something outside the box.

[idanaltman]

thanks for the reply
I'm using pro tools 10 with RME multiface connected to my deskstop computer (running windows 7 ultimate) through an HDSPe PCIE card, the only things connected with USB are Ilok, keystation 66 and behringer BCF 2000, I just tried disconnecting both keyboard and controller and the Hum is still there.
again this hum is not present when PT is closed, only when I open a session with ca 20 tracks...

[idanaltman]

anyone experienced this too?