Why are vehicles so silent?

I just had the worst instance of this happening in BF 6 earlier today.  

I was taking small arms fire on an objective, ran and jumped into cover behind a wall. Thought I heard footsteps on the other side of the small wall I was hiding behind, peeked the corner to see the enemy infantry I just knew was there and I was staring down the barrels of an AA tank that had somehow rolled up from outside the objective without making any sounds. 

Even if the vehicle is stationary, the engine should at least make a sound so we know it's there.