air cooled 2 strokes are particularly noisy with their clicks, clunks, and knocks. What you're hearing MAY be normal "blaster noise" but it may also be excessive... there is no one certain thing that always causes a knocking noise, it can be a whole bunch of things but there are certainly a few things that are more likely to cause a knock than other things.
The most common thing that causes a knock is cylinder to piston clearance. It means it's time to freshen up that top end. 2 stroke engines needs top end attention occasionally. 4 stroke engines have dedicated lubrication which coats the piston skirts and keeps them happier longer but when something does go wrong 4 strokes are considerably more expensive to repair ONCE something goes wrong. Blaster engines have 9 overbores possible so if it is cylinder to piston clearance it may just need overboring.
The noise could also be a crank main bearing. Most often the main bearing flies apart after a lubrication loss. The big end bearing is a roller needle bearing with thrust washers on each side. It can handle not being immersed on oil but it cannot handle not having any oil at all. If the previous owner had a bad habit of running the engine until the quad ran out of gas, it can slowly damage the main bearing by running it for a few seconds each time without oil.
The crank main saddle bearings (or "case bearings") can also be damaged much in the same way that the crank big end bearing can be. They're not needle bearings but cannot stand to run without being coated in oil and will be damaged and get very noisy.
