Engine Rattling

Hey guys I recently purchased a 1982 csr305 belt drive. The bike starts up and idles great no issues going through the gears but I notice once the engine is hot and I've been riding for a while that there is a rattling noise coming from the engine. it almost sounds like it is coming from the drive side. I notice it more when I am decelerating. More so when I close the throttle and downshift. Any ideas what that would be? I checked everything I could think of for something loose like the headers engine mounting bolts etc and everything seems right. only 16000 miles and I changed the oil and filter when I bought it.

Thank you,

Kinda hard to tell from afar. There are several possibilities.
What weight and type of oil did you put in it? Is it the recommended wight? Made for motorcycles with a wet clutch?
Is the drive chain in good shape and properly lubricated?
Carburetors out of adjustment can make a racket, but it is usually most noticeable at idle.
Could be something loose or worn inside.

My thought would be the drive chain guides or the primary chain inside the clutch cover (if that model has those), but definitely check the stuff TexasKZ mentioned first as all of that is way easier to check on.