Take a look at the motors on your drone. If they look dented, the motor bell (the part that spins) may need to be replaced. Also, physically spin each of the motors with your hand to listen for any gritty/grinding noises.

If you hear this, it could mean there’s debris in the bearings. The motor bell and bearings can both be replaced by removing a screw/C-clip on the bottom of the motor.

Once this screw/C-clip is removed, the motor bell can be removed allowing access to the bearings, which are press-fit in place.

The copper windings of brushless motors are enamel coated, so the motors themselves are technically waterproof. With the motor bell off, you can spray some compressed air on the windings followed by a rinse under some warm water to clean out any dust and dirt.

Replace the motor bell and listen for the grittiness by spinning it with your hand again. If the motor spins smoothly, you're probably good to go. If not, try a new motor bell and if that doesn't work, move on to the next step.