Backfiring. Check mixture & ensure firing on all cylinders.
Sloppy timing chain?
Sounds are impossible to CORRECTLY diagnoise unless you hear them. Take it to a garage.
Take it to a shop or mechanically inclined friend.
That you need to take it to the bike shop and take a look at it.
Because of the type of engine, and how the exhaust is made. If take away the silencer on the exhaust pipe, you will get a whole different sound. And If the engine has for example a four cylindre, it will make a nicer, and not so deep sound. With a 8 cylindre engine , it will make a more dark and "groggy" sound.
airplanes make turbine power going up at take-off.In mid-air it makes a turbine at normal speed.At landing it makes turbine cooling down sound.
Sound travels slowly through gas because the particles in a gas are further away then the particles in a solid or a liquid. This makes it take longer for the sound waves to travel through the particles in gases because it takes longer for them to collide with each other; this makes it take longer for the particles to pass on the sound vibration.
You really are going to need to be more specific. What kind of noise are you talking about? Are you asking why noises come out of the engine while operating normally? Are you talking about an irregular noise that you think could be a problem? Engines make noise because there is a conversion of one energy to another and since a lot of it is wasted energy conversion we observe "waste energies" if you will. Such examples are heat and sound. Inside a running engine there is a lot happening. Coolant and oil is constantly distributed throughout the engine and results in radiant heat and sound. Pistons are for ever going up and down, adding more sound. Combustion, which makes the pistons move, adds to the heat and sound. Parts that take this power of combustion and deliver it to the driving wheels are moving parts and add to heat and sound. Everything makes noise. Appreciated properly, all these wasted energies add to the beauty of the beast that is an internal combustion engine.
The cam chain on xr250's are adjusted automaticly by a mechanism inside the engine, so there is no way to manually tighten it. If your engine is making a rattling sound, then your cam chain may need replacing as they do wear out over time which makes them longer
Yes, the word "tag" contains the short vowel sound /æ/. This is because the "a" in "tag" makes the /æ/ sound, rather than a long vowel sound like in "tale" or "take".
u-joints on the drive shaft
Take it to a shop and get the engine serviced. Kinda makes sense if you think about it.