my shock bushings had a ton of slack from the day I bought it..
nothing good about the shocks at all.. no damping, the springs are super stiff, the bushings are garbage

another thing that has been mentioned is to check all the bolts..
what I found on mine is the suspension bolts all have castle nuts and cotter key to hold them, the problem is the cotter key hole is drilled too low. Because of this the castle nuts unscrew a bit before the cotter key stops them.. this causes loosness in the stearing and suspension.

To stop this I got a bag of washers at the hardware store (10mm?) and added 1 under each of the suspension nuts (all the tie rodes, etc.) then I tightened them up and put the cotter keys back in. It tightened the suspension a ton.

Before I did this the steering was always clunky and loose, I could turn the handle bars at least an inch before the wheels would move and now there is no slack.. a great cheap upgrade!

I have also had my exhaust come loose where it mounts under the seat, you might check that as well.

 

The chains on these chinese bikes is actually #40 machine chain and it only comes in 10 foot lengths so you get 2 full chains out of that.

Also, on our 150, the right side lower engine mounting bolt shook loose and fell out while riding causing the torq to pull the engine into a slight leaning position. Then the sprocket started clicking and then it would throw the chain which then caught in the rear end and snapped. The chain then clicked and fell off again, not knowing the engine was leaning to the side. We pried the engine back up and replaced the engine mounting bolts with good quality ones and now it's better than new. The sprockets are aligned again and the chain runs smooth and everything just feels stronger.

And I should add when this bike was new, all the rear end bolts were very loose and needed tightening. The whole rear axle was loosely moving around.

 

mine uses a 420, but they are super cheap

when I changed my sprockets my new chain cost me ~$8 and it came in any length I wanted, I just googled it and bought it off the internet from some random bike parts store

 

Mine was doing the same thing after a few rides. I was also afraid it was the bearings. It was an easy fix. Adjusted chain tension (1 inch of up and down play) and used moly based chain lube on the chain. Make sure all sides of the chain are completely wet with lube. This stopped my clacking/grinding noise completely.