My sr125 moped cuts out at full revs,makes a funny noise first,like rattling

moving noise is very hard to find.

could well be exhaust shields,
did you take a rubber mallet and tap the parts (exh, end to end?)
bang ,rattle, bang , repeat
seen many bad CAT (brick inside loose) rattle. hit it. bingo. there you go.

i also jam objects in cat shelds to stop them for making noise.
just for temporary to find source.

I reckon you should toss it out as you don't really need it as it makes the air damp and mold will start forming inside the walls and create a health hazard.

you say there is a noise on the valve , do you mean the inlet and exhaust valves ? if they are making a jingelly tappy noise then they do need adjusting ! does 1st to third gears rev to near enough max revs in each one or do the faulter and miss ? if miss then have the carb and fuel tank cleaned as well as the adjustment of valves ! exhaust could be choked up also if you only do town runs also , may need decoke of exhaust port and exhaust system !

l think due to drive shaft problem

Could be the carbs are out of balance as this will cause lash noise at the primary drive gears as these are straight cut gears.

You have replaced the fuel filter? This is a ford right?

take it for a full service, clean it, check your brakes r not to tight, check all your oil levels, clean your ait filter ( all done in ur service), check your bettery isnt flat, all that kind of **** n hopefully it will be a smooth runner again

TRY REPLACING THE SPARK PLUG. CHECK TO MAKE SURE U R USING THE CORRECT RATIO OF OIL TO GAS. THE POINTS MAY BE OUT OF ADJUSTMENT.

explain rattling noise. you can take off the fan belts. start engine briefly to see if engine stil makes noise. that will tell you if its engine or the acc.

Glazed alternator clutch plates? Kind of a ratcheting sound? Common when oil wasn't changed or sitting unused too long.
1. Easy fix - drain oil, remove filter. Add diesel fuel to crankcase. Turn engine over. let sit and drain. Refill with new nonsynthetic oil and replace filter. Warm up. Drain, remove filter. Refill with fresh nonsynthetic oil, and install new filter.
2. Harder, but better. Remove plates and clean or replace them.