It can't be a drum because it is clearly affecting all colors.
So open area 2 (green lever) and check the Fuser for marks that match the void areas (I don't think they will be there honestly)
If nothing is found in the above, it will probably be the Transfer Belt. Which is typically replaced by service (I don't think Xerox sells them as supplies, but they are available online)
Whoops, sorry forgot to mention the printer is a xerox c75 (xcc75) printer and I use toners.
I would assume that you will see that on whatever part is responsible, probably some part like a Transfer belt, but since you neglected to mention anything about what printer it is, or even if it is toner or Solid ink, anything is a guess at best obviously.
I've been printing on vellum no problem for awhile and now, all of a sudden, I seen white lines/folds on my tabloids.
I have tried changing the media type, weight, etc., and still shows the streaks/folds.
Below is a picture of what I mean:
Any thoughts to what is the problem???