Okay, here's a strange one. While trying to work out the cause of 1KHz hum being picked up by my guitar, I found out that one of my tracks is also emitting a 490Hz hum (plus harmonics). The weird part is, this happens when playback is stopped, Input Monitoring is off, record is off, there is no input assigned, and the only plugin in the track is the spectrum analyser. It also exists if I turn the FX bin off entirely, although it's inaudible - I just see the peaks appear in Track View, registering at about -129dB. It's not usually noticeable, but (a) it shouldn't be there at all, and (b) when I push 80dB of gain onto the channel via my usual plugins, it becomes audible. The only thing that stops it, is muting the track, or switching off the audio engine.
 
Also - it only seems to happen with certain audio tracks. They are tracks that I have recorded onto in the past, and which have clips in them. But right now they have nothing assigned (as noted above). I've not tried just copying everything into a new track yet, but I'll be doing that if I can't find any other source of this noise.
 
Any suggestions?

I know you mentioned turning off the FX bin,  but what about PC?  I once spent an afternoon trying to find the source of annoying hum in my studio, only to find it was the tape emulator in PC deliberately adding noise..!!

Is it still there if you entirely remove the plugins?  I had a similar exerience with a waves plugin called sound shifter a while back. Bypassing didn't help, but it went away once the plugin was removed. Even more bizarrely, the hum did not present in the rendered final, only whilst editing with the plugin loaded onto the track.
 
Could be completely unrelated of course.

I had something similar once.
 
Tracked it down to having my guitar plugged in > Pod > Interface > Sonar > Input Echo switched on.

Yes, the signal is still there if I remove all the plugins (although I can only tell that from the peak meter - it's too quiet to be audible)
 
However, the culprit DOES appear to be the ProChannel. I never use the ProChannel, and haven't (intentionally) done so this time, either. But when I opened up the strip, and clicked Global On/Off at the top, the noise stopped.
 
I then turned it back on, switched each of the modules off on, and found that the culprit was the compressor at the top. Turning that from 100% wet to 100% dry stopped the tone.
 
I can't reproduce this on any other tracks; but I can switch the tone on or off via the ProChannel for both my affected tracks. Very strange, and not very welcome!
 
Unless there is some sort of vintage compressor that randomly emits a 490Hz hum, I'd say this was a bug. Is anybody else seeing behaviour like this?

I had that problem one afternoon. It was humming and sounded like a phase shifter with an unhooked guitar plug in it. After about 30 minutes of tracking it turned out to be VB3, my B3 plugin. It turned out that the wind level on the leslie cabinet was causing the noise. The expression pedal and volume were rather high and the organ sim was humming thru the leslie with the wind level up high. Sometimes I wonder if we don't get too carried away trying to make softrware instruments TOO real!

I've had that with VB3 also. Still love it though.

I have had this with a plug-in problem before.

Kylotan
Yes, the signal is still there if I remove all the plugins (although I can only tell that from the peak meter - it's too quiet to be audible)
 
However, the culprit DOES appear to be the ProChannel. I never use the ProChannel, and haven't (intentionally) done so this time, either. But when I opened up the strip, and clicked Global On/Off at the top, the noise stopped.
 
I then turned it back on, switched each of the modules off on, and found that the culprit was the compressor at the top. Turning that from 100% wet to 100% dry stopped the tone.
 
I can't reproduce this on any other tracks; but I can switch the tone on or off via the ProChannel for both my affected tracks. Very strange, and not very welcome!
 
Unless there is some sort of vintage compressor that randomly emits a 490Hz hum, I'd say this was a bug. Is anybody else seeing behaviour like this?

Are the preamp gain settings higher on the problem channels?

I don't know what you mean by 'preamp gain' as I don't see any such setting in ProChannel. I do see that ProChannel has defaulted to 'on' in several cases, and that with an input setting of -0.2dB and an output setting of -0.4dB it's not even attempting to be transparent.
 
I went through the project and saw that a whole bunch of tracks actually had the ProChannel enabled, so I went through and switched them all off.
 
This is so annoying because the possibility of having audio-effecting settings in a panel that I never see or view is precisely the reason I don't use or want to use the ProChannel.