The ball is making a knocking noise during operation...

Reason: The ball has suffered a hard knock or drop
Solution: The sphere needs to be opened to examine which of the two halves have been affected.
 
Detailed Explanation: Powerball is a finely tuned product which has been designed to run smoothly and quietly at all times of its operation and this knocking noise is certainly out of character with the product.
 
There is one likely cause and we can go through it as follows:
 
Whenever the unit is dropped (or slammed) onto a solid surface it can sometimes result in a consistent knocking noise which increases in 'pace' as the rotor goes faster and faster.

The hardened steel axle of the rotor is secured in a soft plastic support band which itself is enclosed in a 2-3mm 'channel' between the two halves of the ball’s shell and is allowed to turn freely within this space (there is also some small up/down movement while the rotor is stationary but this disappears once it is in motion).
 
What can happen during a fall or a hard knock is that the steel axle can come down hard on the support rim of either half of the ball’s shell and cause a small dent on its surface. This in turn will present a 'rut' for the axle each time it 'runs' over it, and will cause that 'knocking' sound that you are hearing (and feeling) during use.

Now, be advised that should this happen to your Powerball there are two options available:
 
1. Any Powerball presenting a knocking noise after being dropped in such a manner will be virtually 100% cured if HELD VERY FIRMLY/TIGHTLY IN HAND and run at a constant speed of 10,000rpm for around 60 seconds (repeat this action if necessary).
 
What will happen during such a session is that the hard (but smooth) metal axle (which made the rut in the first place) will literally 'iron-out' the rough edge of that very same rut (or ruts if there are several) and in remarkably quick time, you'll be left with a Powerball that is almost as quiet and smooth as it was in the first place without having to take any further action.
 
When I say 'almost' I mean that you may still hear a very faint noise from the ball each time the axle meets the slight undulation in the normally perfectly smooth surface on which it is meant to run - but be assured that this too will pretty much vanish over time. The key, and I'll stress it once more, is to hold the ball really tightly in your hand as it spins - this is vital to the smooth running of your powerball now and for years to come.
 
2. If, after a week or so of hard spinning (at these high rev's), you are not left with a 'silent ball' once again, then its down to replacing whichever section of the sphere has that 'rut'.