Greetings!
First I'd to say X3 is working great for me (thanks much for the Aria player fix!).  However, there's a click whenever the tempo changes.  Makes for some interesting rit's (can't write the actual word as it gets flagged) in my symphonic stuff:-)  Never encountered this before.  Any thoughts?  I mute everything, it's not there.  I un-mute any track, it's there.
-rich
(X3d)

Perhaps a tempo based effect like a delay? I believe tempo based plugins can cause problems with tempo changes.

WDI is probably correct.  This will be due to some plug-in that has tempo-sync enabled.  Go through all the plug-ins and disable them one-by-one to find the culprit.  Some tempo-synced plug-ins do much better than others when tempo changes.  If the problem is with a chorus plug-in, for example, see if you have another on hand (or find and download a freebie) and see if that fixes it.  Also, if the tempo changes are not drastic, try setting the plug-in to non-tempo-sync and manually dial in an appropriate time parameter value.  That should clear things up.

Tempo changes can also play havoc with synth patches which use built-in tempo synced effects.

Hmmm..  Good ideas.  The cause was Altverb 6 on the master bus.  But I've been using Altverb 6 for years:-)  Could anything have changed in X3 to cause this to show up now?
-rich

Sometimes syncing effects have a sync option, sometimes represented by a check box. If turned on it uses the beats from Sonar and during tempo changes they can't adjust the tempo smoothly to match the bpm changes. Turning off syncing in the effect would fix the issue, but you would loose the tight bpm sync. So perhaps you didn't have sync turned on? Just guessing.

Plenty of things could have changed with X3. Nevertheless the fault lies with the plugin or the plugin configuration or parameters.

CakeAlexS
Plenty of things could have changed with X3. Nevertheless the fault lies with the plugin or the plugin configuration or parameters.

I don't think it's a question of who's at fault. I think it's just the nature of bpm synced effects. I believe even hardware acts this way. For instance the Korg Electribe has bpm synced effects that sound horrible during tempo changes when synced to sonar.

So it's either the way Sonar sends tempo change data or just the nature of bpm synced effects. I'm guessing it's just the way bpm synced effects behave during tempo changes.

I totally disagree with Alex.  When something routine works just fine for the last 5++ years, then stops because of a new release, -- not good.  With this release the Aria multi-player didn't load (fixed, thank God) and now this.  People could loose money on problems like these.
 
I see no sync type option and I'm not using Altverb for anything fancy like beat count delays or anything.  But I welcome further thoughts :-)
-rich

As most if not all sofware has dependencies, saying it should still work just because it has worked in the past sadly has no real logic to it (sorry), pair old software with new software and you are more likely to have compatibility issues, that's just the way it is. What your issue actually is yet to be proven, but it is plugin specific.
 
Oh and sure what WDI states is a fair point.
 
Well you may be eligible for altverb 7:
http://www.audioease.com/Pages/Altiverb/upgrade.php
 
If you are not eligible you may want to try the demo to see if it resolves your issue.