Under the air filter box, above the engine crankcase, behind the belt/clutch housing.
Knocking is usually associated with worn main and rod bearings.
Mine did the same, mine was a motor mount.
it could be your valves or your engine
Knocking as opposed to pinging or ticking, usually means seriously worn bearings of some sort. Time for an overhaul.
2 cycle engine oil. I prefer to use a synthetic designed for ATV's and Snowmobiles. It doesn't smoke as much and stinks much less.
knocking noise
The combination of fuel still being pumped into the engine and air
It depends on a trailblazer 250 they have no crankcase oil but on a 400 water cooled scrambler they do so yes you need to change the oil with regular motor oil
Loud knocking in an engine is usually associated with a rod bearing. Rods connect the crankshaft to the pistons. Have it repaired right away because when a rod goes out it can go through the side of the engine block.
Most likely needs a tuneup done. Check the timing and change the normal tuneup part.
engine knocking hard,and shaking a bit