Worn or chipped gear teeth that will not mesh properly. Shiny wear marks (loss of hard facing) are often visible.

Worn bearings will show up as vibration and noise, as flat spots or cracks appear in the bearings.

Slackness between worn or loose parts - This appears as rattling noises, squealing drive belts, "piston slap" in motors, air leaks, etc.

Poor lubrication - This appears as squeaking noises due to friction or excess impact noise in dry and worn gears or bearings, and universal joints.

Imbalance and poor alignment of rotating parts - any imbalance in a fan impeller or motor shaft will show up as excess vibration.

Obstructions in airways - A build-up of dirt or a bent/damaged piece of metal in an airway, or near a moving part (e.g. a bent fan guard), can cause whistling or other "air" type noises.

Blunt blades or cutting faces - Blunt or chipped saw teeth, drill bits, router bits, etc., usually make the job noisier as well as slower.

Damaged silencers - Silencers for air driven machines, or mufflers for engines may become clogged with dirt, rusted out or damaged, losing their ability to absorb noise.

Removal of a noise-reducing attachment - Mufflers, silencers, covers, guards, vibration isolators, etc., which reduce noise should never be removed except during maintenance, and then must be replaced.

Loss of effectiveness in door and window seals and insulation in general  e.g. excessive clearance around coax and electrical cables.This allows noise to be transmitted