Export Audio and Render in place giving different results

I have a simple test song, using 2 soft synths only. I have recorded the mdi and rendered each of 2 tracks to audio. I left 2 bars silence at the start. This is total silence, nothing registers when this plays back, tested on Span metre down to -180 db.

I exported the song : no inserts, same 44khz/24 bit settings, and the exported audio sounds the same but…in the silent 2 bars now I see some action low down in Span. Something seems to have been added in the export process.
If I normalise this activity in the 2 silent bars it sounds like a harsh buzzing sound.

If I use Render i Place, these 2 tracks rendered inton one Audio track, I do NOT get these artifacts low down, its totally clean.

Why the artifacts? And why the difference?