New roundwounds are brighter and ring longer than old strings, so a small buzz that was always there can suddenly be audible. Plugged in, it may not matter. If you hear it through the amp, it does.
Check, in order:
- Seating: each string should sit in its nut slot and saddle groove. A ball end not fully seated can buzz.
- Length: the full-diameter winding should run past the nut, and a taper shouldn't sit on the saddle unless the set is designed for it.
- Gauge change: lighter strings pull less on the neck, so relief can flatten and cause buzz on low frets. Heavier strings can do the reverse.
- Construction change: flatwounds and roundwounds of the same gauge feel different under the fingers and can change relief.
- Saddle height: low action buzzes more with a strong attack.
Give a new set a day to settle, then adjust the truss rod in small steps if relief changed. If buzz persists in one spot, the fret may be high. A technician can level it.