Anyone else got this.
 I thought my mp3 encoder wasn't working but it is.

I had some problems with an audio "artifact" at the beginning of a track playback.  It turned out the Pro Channel was enabled and the Breverb2 in the pro channel was also enabled and the Breverb2 was causing the artifact.  Disabling the Pro Channel or just the Breverb2 fixed the problem.

I wasn't exporting MP3, just playing an imported audio track, but something to consider.

I'm importing a stero wav mixdown into a blank project - nothing else. It's not artefacts I'm hearing it's a noise pulse.

Interesting to hear your comments though because I have been having minor crackles and pops when mixing down a project started in 8.5 which uses Breverb and pro channel

Well, I was importing a stereo wav file, too.  I had some older mixes that I wanted to import and add Ozone 5 mastering to and then export as wav files. 

You may have to remove the fast mixdown and see if that takes care of it.  

Noise/pops/crackles, etc are a major complaint I see on other computer forums.  Many causes.  Run the DPC Latency Checker and see if shows up anything.  http://www.thesycon.de/deu/latency_check.shtml