There appears to be an odd design quirk with the PS4. If you place the console horizontally on a flat surface, you’ll notice it wobbles slightly.
The reason for the teetering motion is attributed to the placement of rubber feet on the underside of the console; there are two fins that run down the centre of the PS4 and another on one edge and they cause the unit to be unbalanced on a flat surface. Some may not see that as a great issue but it can actually cause a lot of noise due to the vibration from a fast spinning Blu-Ray.
I resolved it by some domed soft rubber feet which I added to each corner. The difference they make is quite extraordinary – the PS4 is a lot, lot quieter in use (well, whilst the Blu-Ray is in use anyway).