Rattling is a common noise with air conditioners, as the various fasteners throughout the machine can eventually come loose. If you hear rattling, start by checking the screws or bolts holding any cover plates in place. Tighten them accordingly. If the noises continue, remove the cover and check internally for the same issue. While you can listen for rattles, you should flip the power to the unit off at the breaker when you actually go in to tighten screws and bolts.
Whistling is generally the result of seals which aren’t doing their job properly. This is most common along the cracks and seams of ducts as they exit the central unit and attach to the ductwork that carries the air throughout the home. Examine the seams around these areas for loose connections and retighten any screws or bolts. If heat tape was used to also cover the seals, make sure the tape completely adheres to the duct seams. Check for flaws in any caulking and re-caulk as needed when you find the voids.
The interior motor of an air conditioner unit relies on belts just like a car motor. Anyone who has ever owned a vehicle has heard a motor screeching from time to time when the serpentine belt within begins to wear out and stretch. The same problem can occur with air conditioners. If there is periodic screeching that only occurs when the unit turns on to blow air or cool the air down, you are dealing with a worn-out belt that needs to be replaced.
Thuds mean something is loose and flying around within the air conditioner unit while still attached to a turning motor. Sometimes this can be something as simple as a fan blade coming loose from a motor and consequently bumping into a guard while it spins. Or it could be something more complex, such as a plastic component or cover plate falling off and into a fan where it causes thudding every time a fan blade passes. Remove the cover plate so you can visually inspect the interior to discover the offending object.