Weird tone after making changes to project

 
Has anybody had an issue like this?
 
I have a project I started in X1 but I'm mixing it in X2. It's only about 30 tracks, most of them frozen midi tracks (sd3, Kontakt, Dim Pro), and not that many plugs. Whenever I make any changes at all to the project, even something as minor as a volume change on a track, when I play the project I have a tone, like a test tone or something, that plays in the background. If I close the project then reopen it everything is fine but as soon as I make another change the tone is back.
 
Another thing; this is the first project I've worked on in X2, I haven't started a new project yet, and my cpu meters are waaaaay up there, like almost into the red zone. Is X2 that much more of a resource hog? This same project never got anywhere close to that in X1. My computer isonly about a year old, quad core @3 ghz with 16 gigs of ram, seems like it shouldn't be straining like this on such a small project

Another possibility is a demo plugin that will not allow you to save changes or has bypassed its demo expiry time.

FBB- All the midi tracks are frozen. I even tried archiving the tracks with midi themselves but it didn't help.

lorneyb2- I don't have any demo plugs installed, all the plugs are Cakewalk plugs included in X2.

Jonesy- Good idea, I haven't run DPC latency checker with the project open, I'll try that today, see if I can isolate anything.

Thanks for your replies, guys, really appreciate it. I'm sure I'll find in the end it's something simple I've overlooked.