Some abnormal bearing sounds & their possible causes. Please note that the description of some of these sounds is rather subjective & thus could vary from person to person.

SOUND FEATURE POSSIBLE CAUSES
Buzz /Roar Loudness & Pitch changes with speed

Poor Fit (poor shaft shape).
Bearing Rings Deformed.
Brinelling.

Vibration of Raceways,Balls or Rollers (for large bearings, if this sound is minor, then it may be normal).

Chatter

Noticeable at Low Speeds.Continuous at high speeds

Bumping in cage pockets (insufficient lubricant). Eliminated by clearance reduction or pre-loading.

Rollers bumping into each other in full roller bearing.
Clatter/Clang

Metalli, loud bumping sound. Thin section, large bearing at low speeds

Bearing ring deformed.Grating of key.

Crunch Felt when the bearing is rotated by hand

Scoring of raceway surface ( regular) Scoring of balls or rollers (irregular) Dust/Contamination.

Deformed bearing ring (partial interference clearance)
Faint tinkle

Irregular (not changing with speed). Primarily on small bearings.

Dust in bearing.
Fizzing/Popping Generated irregularly in small bearings Bursting of bubbles in grease.
Growl Continuous at high speeds. Scoring on raceway, ball,or roller.
Hiss Small Bearings. Raceway, Ball,or roller surfaces are rough.
Hum Disappears when power supply is switched off. Electromagnetic sound of motor
Large sound pressure Large sound pressure

Rough raceway, roller, or ball surfaces.
Raceway, rollers, or balls are deformed by wear.

Large clearance due to wear.
Rustle (intermittent) Generated intermittently at regular intervals

Chafing at the labyrinth.Contact of cage & seal.

Rustle

Sound quality remains the same even if speed changes (Dirt.

Sound quality changes with speed ( Scoring)

Dirt. Raceway, ball, or roller surfaces are rough.

Rustling patter Regular & continuous at high speed.

Generated by retainer. Normal, if sound is clear.

Grease is inadequate if sound is generated at low temperatures ( use softer grease). Wear of cage pockets. Insufficient lubricant. Low bearing load.
Screech/Howl

Occurs mainly in cylindrical roller bearings.
Sound changes with speed.

Loud metal sound that disappears temporarily when grease is added.

Large radial clearance.

Poor lubrication or grease consistency.
Squeak

Metal-to metal spalling sound.High pitch.

Spalling of roller & rib of roller bearing.
Small clearance.

Poor lubrication.
Squeal Generated irregularly due to grating.

Slip on fitting surfaces.Grating on mounting seat, on key, etc.