Audio artifact that isn't really there

I was recording some overdubs for a band this week and during playback we heard an audio artifact (a click or sqwelch) that we've never heard in any other mixes on this song.  It is repeatable every time when playing back within Sonar, but the original wav files don't contain this click, mixdowns don't contain it.  Soloing each track individually, you can hear the click.
I thought it might be a hard disk throughput issue but there aren't that many tracks (24).

It is just so repeatable at exactly the same point in the playback, if I start from the beginning or half a bar before.

I'm convinced if I load all the tracks into a new project, everything will be ok. Except then I have to re-create all the levels, fx and settings, which is quite a big task.

Any ideas?

Sonar 8 Producer Edition.  Windows 7 (32-bit).  All standard plugins, no VST instrument tracks, all audio.
Tascam 1804 Firewire sound card.

That was it.  Thanks Ace. 

I have been recording with ASIO latency at 128 (bytes I presume) for a couple of years without any problems.  

With this particular project, the click goes away at 1024.  I tried the other settings (256 & 512) but they give the same problem.

The mixing latency page in Sonar is all greyed out so I can't change it, but it shows 2 buffers in playback Q and effective latency is 2.9ms.  Which, granted, sounds a bit small

Just so you know, It doesn't matter what the buffer size is as far as recording goes too. Only if you try and use real time effects or Guitar rig. Sonar adjusts the latency so all is still synchronized perfectly no matter what the Round trip latency is.