more than likely, if it emanates from the muffler, and its rattly sounding, tis just a loose baffle. irrelevant. you cant damage much by removing them anyway. just possibly your hearing?
usual trick is remove the piddly lil 10mm outlet and drill it out to take 13 or 16mm or so pipe. your bike guy says anything about backpressure and advises against it? he isnt such an expert. he hasnt fixed your simple rattle
you can examine it to a degree by removing the nut/bolt at the end of it... if present.
its also a twostroke, sure its not just the sound its meant to make? not normal with the stock exhaust, really, that ding ding ding thin wall tuned pipe "marbles hitting a tin" sound, but?
everything else is just as possible.. the clutch cover buzzes like an angry hornet, and the gear mesh is far less than desirable. like they missed a tooth on one of the gears... or the centres are wrong. these engines are just noisy to start with
hmmms. he took the top off? whole cylinder or just the head? could be dinky lil end bearing.
real way of eliminating the muffler as a problem is to eliminate it... wear some earplugs and really, you will be amazed at how many of the "smaller" sounds you can actually hear... even the chain squeaking... if the sounds gone, its in the muffler, just a loose baffle, no major concern, put it back on and live with it
as for rims and wheels. spoke-key and clothes-peg. start truing
so easy if you just stop and look at how a wheel is put together. or search the net. wheel building is an easily learnt art.
you can tension the spokes properly so the wheel doesnt collapse. while ive amazed myself by how bad i can let them get, every loose spoke is a section of rim that is unsupported, therefore has no strength. the strength of a wheel is more than the sum of its parts. its fitting them together properly that does it
yes, of course, the better the parts are to start with helps.
my main preference is that the rim im scavenging starts off fairly straight. even found one with 36 valve holes
seriously, theres a hole between each spoke... had a pair once. musta threw one out.
last thought. engines nice and tight and solidly secured, and the buzz isnt in the frame, possibly?