air compresses and the oil doesn't (pretty much). so if there were tiny air bubbles in the syringe and you pull back on the plunger to aspirate the air bubbles will expand and become noticeable. when you stop pulling the plunger it will go away unless you pulled so hard you sucked air down the sides of the rubber plunger or where the needle and syringe come together (always make sure that's snug). when pulling into the syringe or injecting don't force too hard or get into a rush. when you aspirate pull with about the same amount of force that you did when drawing the juice from the vial. if your using a thin needle,23 or 25g, fluid moves slowly through it. that's why when aspirating you do if for a couple seconds, you don't just pull hard as hell.