I had a similar problem when a mate of mine thought he'd 'tune my engine' for me. Ran fine at idle and with no filter, as soon as the filter was back on and under load I got very similar symptoms.
I replaced...plugs points leads distributor cap timing back to stock and now it runs fine, I also adjusted the tappets and mixture while I was at it.
It may not be the end

if it doesnt make a noise when you are standing still and revving the engine, it may be something in your drive train....

first thing that i would be checking is that the tailshaft bolts are all still there and that the tailshaft has not had a knock and gone out of wack ???? or for that point the yoke doesn't have any play in it ????

i once had a similar problem for weeks and turned out that on a clutch change i didnt have the thrust race bearing on properly and everything went out of wack

"food for thought"

Cheers
Peddler

A trick i used to use on my mountain bikes (and motor bike once or twice) is to get a screw driver or metal rod and put one end on where you think the noise/vibration is coming from and the other end in your ear or against your head near your ear. The closer you are to the problem the noisier it is and the clearer it is.

I've found that with my engines you can just use your hand to feel the vibrations as they get stronger, the closer you get to the problem (no need to stick the screw driver handle against your ear).

Doing it with cup and cone type bearings i used to be able to tell wether it was the ball bearing that had shat them-self or the cup/cone just by the frequency of the sound.

i had the "death rattle" come from nowhere there was this tiny noise like a loose screw in the dash or something and then bang Sh!t hit the fan i scored the 3rd cyinder burnt rings i killed that sucker. so check it out as soon as possible

I adjusted the fuel mixture on the carb and the noise seems lower or sometimes i cant even hear (leaner), and when I adjusted it back the same way as yesterday (rich) the noise is louder...

Could even be a big end bearing known as the "death rattle", sometime you wont heart that rattle till the revs come up a bit and the piston starts smacking the #### out of the heads.

Whatever it is it doesn't bode well

It doesnt make the noise when its parked and just revving the engine...
it makes the rattling noise when you push the pedal half way down to fully down and accelerating. The noise goes away if you lightly just push the pedal down while accelerating.

have you worked out where the rattle is coming from yet? from under the rocker cover or where?

I notice that I have to keep puting more oil after a month, but there's no leak or anything...
might be my pistion or head gasket being really worn out?

when was the last time you checked your oil?