Knocking sound coming from freewheel? (Especially form when riding uphill).

[Mountain]

Hi.
I have a fairly cheepish bike, Appolo Evade Mountain Bike. (The reason for it having a freewheel was probably due to price not age).

Anyways. So, there is a tiny bit of freewheel wobble which I have come across on a lot of freewheel bikes so I never considered this a problem. Freewheels just tend to get a little bent right? Despite this, shifting is just fine and smooth and the whole drive system is just good overall. However  

When I ride up a hill, and I mean steep hills, doesn't matter what front/freewheel gears I'm using, a knocking sounds starts to develop and then stays there regardless of flatness of the road. I've come to find changing in to the lowest gear 7, the noise quiets down a bit a may stop making the knocking sound *after* riding on a flat for a while. It is also quiet on 1 and 2. It's very loud when using gears 3 through 6. I assume that this happens because of the either bent/bad threading on the freewheel hub (or cone thing, sorry not sure what it is called).

So, could the reason for this a bad hub thing. The thing that slides through the whole hub and onto the freewheel itself?
I wouldn't of had an issue with the freewheel wobble, it's very slight and I've come across this before without actual performance issues, but this knocking sound is odd. I'm 99% sure it is coming from the freewheel cus the sound changes when going through the gears.