Motorbikes talk constantly — most of it is normal mechanical noise. The skill is noticing the new sound, the changed sound and the bad sound. Diagnosis by ear catches problems at the cheap stage; ignoring sounds is how small ones become engine rebuilds.
The vocabulary of noises
Tick-tick that speeds with the engine: often valve clearances on bikes with adjustable valvetrains, or an exhaust leak — workshop-checkable. Squeal that happens braking: brake pads announcing themselves — brake-checks-motorbike-vietnam covers the full picture. Squeal or clunk from the left side when rolling: chain and sprockets — loose, dry or worn; chain-maintenance-motorbike-vietnam covers it. Rattle from plastic bodywork at idle: usually just panels vibrating, commonly fixed with a hand-tightened screw. Hum that grows with speed, felt as much as heard: tyres or wheel bearings. Knock under acceleration, deep and metallic: engine internals — stop riding and get a mechanic, today.
The reading method
Find when it happens: at idle only, with revs, while braking, on bumps, on corners. Noises that appear only when moving but vanish when braking gently are usually rolling hardware (wheels, chain); noises that track the engine regardless of speed live in the engine or transmission. Where in the rev range it appears matters too — a tick at idle only is far more relaxed than a knock high in the rev range.
When a noise means park it
Metallic knocking, any grinding, a screech with smoke, or a noise joined by a new vibration or a warning light: stop. Continued riding is how noise becomes damage. What to do next on a rental — reporting it to the shop rather than self-diagnosing — is covered in rental-motorbike-feels-wrong.
The honest limits
Ear diagnosis narrows a problem to a system, not a part. A mechanic with the bike in front of them finishes the job — motorbike-service-where-vietnam covers finding one worth trusting.
The one-line version
New, changed or bad is the filter; where-and-when is the clue; grinding and knocking mean park it — bikes always tell you before they break, in a language worth learning.