Not sure if i should try to get a new tire?
My vote is for flat spots. Any vehicle with pneumatic tires will generally develop flat spots from sitting too long. 24 hours might not be enough to develop severe flat spots though, but a few days to a week and I notice them on an old Saab I don't drive every day. The flat spots diminish the longer you drive and are unnoticeable after a half mile or so.
Just guessing here but if that is your problem, store the bike with the wheels off the ground. A hard plastic milk crate is good, or wooden blocks or even a padded cinder block (so you don't scratch the paint) placed under the center of the bike.
Could also be low quality Chinese tires. They could be out-of-round or lumpy looking as they come out of the mold, or from improper storage.
As for making it quieter, most people keep the stock box muffler but you might try experimenting with something custom. Anything with the proper outer/inner diameter could conceivably be used. Unfortunately the stock muffler just can't be sawn off from the short exhaust pipe that it is attached to, it is not long enough. Look around for a 1 inch diameter mini bike exhaust pipe/header and then a 1 inch or metric 25-28mm mini bike/pit bike/dirt bike/quad, atv muffler and experiment. Bigger is better but too big can stress the exhaust mount on the engine so make certain the muffler has a bracket to mount it to the frame.
On ebay I ran across several Dirt Bike, Pit Bike, Quad, ATV long style, cylindrical mufflers that might work.
There's probably more than a few examples here on the site, but this is basically what I mean (with or without the header wrap):
https://www.oldminibikes.com/forum/index.php?threads/custom-alum-muffler-and-wrap.64561/
