Why is this sound happening in my rear wheel?
 

youtu . be/wcDseFuvZFk
youtu . be/ljoLdVD43Ec

I also noticed the chain starts moving slightly when the wheel starts slowing down.

Please post a valid URL. Preferably, one that links to the correct video.

Sorry, it won't let me. Remove the spaces between the dots, its a shortened youtube url

It seems like the freewheel. It seems unusual. I can't say for sure.

I would start by running oil into the freewheel, and see what happens.

Does it function 100% while riding, apart from the noise. If it does, and oil doesn't fix it, you could pull apart the freewheel and have a look, or replace the freewheel, or just ride it as is.

Is this a new freewheel? What alo said, run some light oil into the freewheel. Also remember that freewheels, freewheel cogs, and chainrings are not perfectly round, especially at lower price points. Try running a slightly looser chain just to see if that reduces noise.

I cannot get them either, youtube says it cannot find them

Yeah, its a new bike I got for cheap.

remove the spaces in between the dots, I can't post links because I'm new

Chain is too tight

Explains the twitching transferred to the cranks. Cheap freewheels work fine with a spring-loaded chain tensioner like a rear derailleur, which, of course, is unnecessary with a single speed. But chain tension binds against the out-of-roundness. I worked in a shop that sold tons of these. Apparently customers knew they were getting cheap bikes so they didn't notice. Loosen the chain or get a better freewheel.

Time to take it apart & look for a bad bearing race.

Yep, chain is wayyyyyyy too tight.

here it is without the chain
youtu . be/A_JOyeW7IgQ

Perfectly normal for a cheap freewheel. The bearings aren't the best quality but then they don't have to be- when you're coasting there's no load on the bearings.

Put the chain back on, make sure it's not too tight (a little variation is normal when pedaling) and ride the bike. It should quiet down as parts wear in.

Lube the freewheel by oiling through the gap between freewheel cover and teeth ring, "run it backwards" with finger to help coating everything inside ( I would continue oiling 'til it runs from the backside gap, but you may stop when/if the sound diminishes; it will not be muted as it being a cheap frrewheel) and test. While you're at it, check the hub for grease; there is this sound like "dry" ball bearings falling onto each other and I'm not sure if it comes from the freewheel or hub.