hi Carbon/sludge
my kids 315r
started to rattle (not very loud)
due to it being 4 years old
i order a piston kit
striped barrel piston ring seized with sludge
cleaned with petrol
problem solved
looked at the 2 stroke oil from the dealer (rock semi syn)
dealer says
i have seen this before using cheap 2 stroke oil
his advise was only run fully syn oil
hope this helps