Usually the noise is caused by either the fan belt that runs the Power Steering pump slipping or by the power steering pump itself having issues.
It might be low on fluid, so check that.
If you want to determine if it is just belt noise: (very carefully!) take a bar of soap and hold it to the belt (both sides) with the car running and see if it quiets down...(this is not a "fix" just diagnostics) DO NOT do this if you can not reach it easily...getting hit by the fan or having hair or clothing or fingers caught in the belts can cause serious harm.
The most likely reason for the noise is a belt that is out of adjustment or become glazed. you can try adjusting it or replace it... maybe even clean it if it is greasy, with a solvent, gently, with the engine off, you will need to turn the engine by hand if possible.
The loud noise from the AC when turning off is often caused by the sudden release of built-up pressure in the system as the compressor shuts down. This pressure release can create a loud "whooshing" or "popping" sound.
Check your power steering fluid. Could also be a power steearing pump going bad
Why should I know?
When an air conditioner makes a loud noise when turning off, it could be due to the sudden release of built-up pressure in the system. This can cause components to vibrate or move, creating the noise. Regular maintenance and checking for loose parts can help reduce the noise.
ball joint or tierod end
If it is a thumping and a front wheel drive car, it is probably one of the axles.
Your furnace may make a loud noise when turning on due to issues such as a malfunctioning blower motor, loose components, or a clogged air filter. It is important to have a professional inspect and repair the furnace to prevent further damage.
It is generally prolonged exposure to loud noise that will damage your ears so look after them and protect them, either by turning down the music/TV if you can or wearing ear defenders, especially if your in a noisy workplace. Avoid loud concerts/parties etc.
Loud noise, especially over a long time, can cause deafness or impaired hearing.
If the sound is a loud growling , whining, noise you're probably low on power steering fluid. Now the cause of the fluids loss is WHERE ?
When an AC unit makes a loud noise when turning off, it could be due to the sudden release of built-up pressure in the system, causing vibrations and rattling. This noise is often normal and not a cause for concern, but if it persists or gets louder, it may indicate a problem that needs to be addressed by a professional technician.
Power Steering Fluid Low.