I noticed that when exporting to a final stereo wave file that in the "silent" part before the music kicks in, if I normalise that part I get some bleeps, buzzes, and sometimes white noise ............ if I export a couple bars of silenece before the music kicks in, it should be genuine silence right?
 
Any ideas how to troubleshoot this - what could cause it?

Am I reading that right? You are normalizing silence? 

What could cause it?
 
Normalising silence (i.e. applying huge amounts of gain to a non-existent signal)

Absolute silence, meaning a string of zero-value samples, would indeed still be silent after normalization. However, that lead-in probably wasn't absolutely silent to begin with, as absolute silence doesn't exist in reality. If nothing else, you've got dither and quantization noise assuring that a silent part isn't a string of zeroes. You don't usually hear it, but normalization is going to take whatever's in there and turn it up. 

Yes, that's what happened - I took a passage which is supposed to be completely silent, normalised it to see if it really was ... and it wasn't (!)  --- this was couple of bars where nothing at all was playing, so I didn't expect to see anything, even when turned up, but I saw a flicker on the master output and that's why I checked.  Its quiet, but when the track is bumped up for mastering it can be noticeable.

Getting rid of those artefacts is part of mastering ( if you know what your doing) 

Those artifacts should not be there in the first place.    I tried various tests, and it seems certain plugins are causing it.  Adding for example Fab Filter Timeless or Fab Filter Saturn to one track then exporting the whole song, in the 2 count-in silent bars at the beginning, I can see there is something way down at -125db, not audible even I guess.   If this is normallised it sounds like a buzzing.   Take way the insert, no buzzing.  Problem is, 50 of these tracks in a full arrangement and that buzzing is getting up to noticeable levels around -80db, and when mastering its getting audible.    
Why would adding certain plugins cause this?   Is there something I can do?

That is normal.
"Normalising" FORCES all ground noise to 0dB. Even if the ground noise is -120dB = absolut silent.
Just tested. Even muted parts will be normalised to 0dB hum.
Absolute silence means -120 dB and "normalising" adds 120 dB to it = 0dB white noise and hum.

Why are you upset about something that most audio engineers are well aware of and has always been there. 
Wth hardware we call it self noise. Crank anything up full and listen with headphones. You'll hear a lot! 
It's one of my favourite tests of the quality of gear like mixing boards. 
Plug in a set of headphones, turn them up and now turn up the pre amp to max. Last time I did this I bought my Yamaha as it was the quietest.
And with a computer!! My goodness there's a lot of noise in there and yes everything you add to the signal chain be it analog or digital will just add more noise. 
I make a lot of live concert recordings of our local choir, the auditorium is a beautiful quiet room. But man , if you try and push the quiet parts you'll soon hear the HVAC and shuffling of paper.
SO it's not exactly a bright idea to apply gain to silence.. you need to apply "silence" to silence.  An easy task in Wave Lab.  Highlight, Backspace. 

Any audio that's been dithered will contain noise. Dithering adds noise, it's how it works. That noise is usually at such a low level it's inaudible or almost inaudible at any 'normal' plygack volume, including club systems. But if you normalise it to zero dB then yes, it's going to be loud. Because you've just boosted it by anything up to a hundred dB or more depending on bit rate.

Any audio processing, even digital, will add noise even if the plugins used aren't emulations of noisy, vintage hardware. Even good linear phase processors add some byproducts in the way of noise or phase shifting which comes out as noise. Again, normalise the noise and it will be very loud.

Software synth's filters etc. almost certainly produce some output even when the synth isn't receiving MIDI.
And you probably don't want to near how much oscillator leakage some very reputable hardware synths produce if you take their 'silence' and normalise it.

What you are hearing is perfectly normal, even if the audio was created entirely 'in the box". If it involved any hardware at all then the noise would be worse.

The motto? Don't normalise anything and expect silence.

If you sufficiently magnify the wave form for what looks like a flat line in any DAW you're very likely to find it isn't really a flat line at all if it's the result of recording or has been processed in any way at all.