Could it be: pedal working loose?

Tighten each pedal with a pedal wrench (left pedal is reverse-threaded).

Could it be: crank bolt loose?

Push the crank arms sideways. Movement at the arm means re-torque the crank bolt.

With cranks tight, play or grinding in both arms is the bottom bracket.

Stop and check before riding further: a loose crank can strip its threads or fall off.

Pedal working loose · Most likely
Tighten each pedal with a pedal wrench (left pedal is reverse-threaded).
Fix: Remove and install pedals
Crank bolt loose · Most likely
Push the crank arms sideways. Movement at the arm means re-torque the crank bolt.
Fix: Remove and install square-taper cranks · Remove/install two-piece cranks · Remove, install and remove play from 3-piece cranks
Worn bottom bracket bearings · Sometimes
With cranks tight, play or grinding in both arms is the bottom bracket.
Fix: Replace cartridge bottom bracket · Replace external-cup BB · Replace Mid BB bearings