There is a loud metallic rattle/clacking noise coming from my exhaust after the bikes engine is hot, basically after riding for 15-30 mins when I crack the throttle at a complete stop in gear or in neutral, doesn't matter, doesn't happen under load, only when I stop and rev the engine. It sounds like the exhaust is loose or something.

SOURCE: When I get on the throttle I hear a metal rattle sound

This sounds like pre-ignition, aka detonation and engine knock. Change to premium gas and put in a set of spark plugs that are one heat range colder than stock. It is good that you are handling this before the engine destroys itself. Yes, destroys itself. That is what pre-ignition will do. By the way, good description!

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SOURCE: Honda CR125 Motocross bike throttle delay

ALWAYS have a fire extinguisher on hand when working on carburetors.
Drain the carburetor. There should be a screw on the lower side or bottom of the carb float bowl. Remove the screw then replace it after the fuel drains. Turn the gas back on and wait a minute for the carb to fill with gas. Now pull the top cap off the throat of the carb and remove the large slide. There will be a long needle in the center of the slide. Remove the needle and then move the clip at the top on the needle down one notch. Two notches may be required but start with one notch. Put it all back together and try it out. Moving the clip downward richens the low end fuel mix.

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SOURCE: 2008 Z750 bad grinding noise in first gear

It is possible that the first gear shift fork in the transmission is bent causing the 'dog-clutch' between two gears to just barely mesh. With engine off, push the shift into first and clutch lever released. Rock the bike back and forth to see if the gearing seems to be jumping in and out. There may be a problem with the shift lever shaft mechanism not allowing a complete shift into first gear. Hope this helps!