Head scratcher

[Imnomechanic]

Hi very new to forums but really in need of some help.

I have a uk 1999 gs 500 its never ran properly since I got it but it was cheap and I needed a commuter. It has 2 problems firstly it has a rattle that is  coming from near the valves it doesn't do it when the engine is cold or warm just in the mid temp (I'm guessing). But as I normally only do 6 to 7 miles to work its always there and getting worse/louder. The second is when accelerating and I want to whip the throttle back the revs die  it chugs a bit then goes.
Things I've done I have change filters, oil, spark plugs and check the valve clearance, I also had a mate clean the carbs out. As the name suggests I'm no mechanic but really stuck here any help gratefully recieved.