Start by finding where the buzz happens. If it changes as you fret different notes, it's probably fret buzz. If it happens on one note regardless of how you play, something is probably vibrating in sympathy.
Check for loose parts first:
- Tuner nuts and screws: tighten gently, without overtightening.
- Bridge pins: make sure each ball end is pulled up against the underside of the bridge plate, not caught on the pin.
- Inside the body: a loose pickup cable, battery bag or label can rattle. Reach in and secure it.
- Truss rod cover, strap buttons and pickguard edges: tap them while playing to find the source.
Then check fret buzz:
- Relief: too straight a neck causes buzz in the first few frets.
- Action: a low saddle causes buzz all over.
- Weather: dry air shrinks the wood and can drop the action. A humidified case helps.
- Frets: a single worn or raised fret buzzes on certain notes.
If you recently changed to a lighter gauge, the strings swing wider at lower pull and may buzz on a low setup. Going back up to a set like P1254 Light in Phosphor Bronze can help.