I have a strange noise/hum issue.  I can tell that it's only happening on tracks that have Melodyne Region FX in them (EDIT: later discovered to also be an issue with Slate Digital Virtual Tape Machines).  I can see the tracks sitting at around -42 dB, and if I mute them, the hum disappears (and the level meter returns to 0). 
 
This is happening when I use my Alesis MultiMix12 Firewire, my Xonar Essence STX soundcard, or my RME Babyface (edited for clarity).  It also occurs with my Presonus FireStudio Project, but is far less prevalent (edited for correctness based on new information).  None of the tracks in question have their input set.  If I open the Preferences dialog, the hum goes away as if I'd muted the tracks.  It seems like some kind of feedback, but with no inputs enabled in Sonar and nothing plugged into the devices I don't see how that can be the case.  Input echo is also off for all these tracks.
 
Anyone faced a similar issue, or have any ideas on how to troubleshoot further?
 
EDIT: additionally if I delete all the clips from my 3 tracks with Melodyne then ctrl-z, the noise abates for about 10 - 15 seconds before gradually returning (over the course of about 5 seconds) to the volume it was before.  Bizarre.

Is there a good reason to use two soundcards? The most recommended driver, ASIO, only supports one card at a time (ASIO4ALL is not ASIO). However, the hum most likely roots back, one way or another, to using two cards, so unless you have a good reason to do so, use only one.
Also using more than one card often causes MIDI clocking and other timing/latency issues. All cards usually work best with their own, native ASIO-drivers. For some WDM works equally well.
 
Secondly, your post doesn't clearly tell how you're using Melodyne, but the rule of thumb and more or less a concensus on the forum is:
Open Melodyne - do the edits - bounce to tracks - remove Melodyne. Do not leave Melodyne in the active project.

This is happening when I use both my Alesis MultiMix12 Firewire and my Xonar Essence STX soundcard. It does NOT happen with my Presonus FireStudio Project.

Well there you go. You solved it!! The Xonar card is not recommended and using 2 different cards is not a good idea. The Presonus is a recommended sound card to use with Sonar, so stick with what works and do not use what doesn't work.
 
CJ

Depending on how you read the post, the OP could be saying it happens when he uses his Xonar card and also when he uses his Alesis. I didn't see any mention of ASIO4ALL.

Perhaps he's using two cards and ASIO4ALL but let's not jump to conclusions here.

He could be using WDM and the feedback noise he describes sounds like a clocking issue. Jumping to conclusions is what we all do best :)
 
CJ

Hey guys, thanks for the responses.  To clarify, Sanderxpander is correct - I mentioned multiple cards purely for testing purposes.  I am only using one at a time, and I'm not using ASIO4ALL.  I'm using ASIO with all devices but with the manufacturer's drivers.
 
I've also found that this problem isn't isolated to Melodyne - it happens with particular plugins.  I've discovered that Slate Digital Virtual Tape Machines is also affected by the problem, which kinda sucks because it sits on the master track and has children on all tracks.  I can always bounce the Melodyne Region FXs to clips (as suggested, and I generally do, but I'm trying to troubleshoot this thing), but the VTM is a more annoying and permanent problem.

So it seems like certain plugins are causing some kind of feedback.  I can't find any literature on this anywhere, and even figuring out the correct search terms for something like this is difficult.  Going to update the thread title and original post with new information, and clear up the confusion with sound card(s).
 
EDIT: additional info - if I switch the RME Babyface from ASIO over to WDM, the problem persists.  So it's not a driver mode issue.   Also, I've just discovered that the issue DOES occur with my FireStudio Project, but it's way less prevalent - to the point where it's hardly noticeable unless you know what to listen for.  But it's actually good to know that it's happening across all devices, in a twisted way...