At the low end of the price range, and assuming that there is no repair or replacement of AC system components required, then somewhere in the region of $100-$150 could be the cost of a regas. However, the cost will depend on the type of vehicle being serviced and other factors.
A rattling noise in your AC system could be caused by the following;
Compressor: If working parts within the compressor begin to fail, this can be the source of the noise.
Drive belt: If the belt is loose or worn it may slip when the compressor is engaged, and lovely noises ensue.
Compressor Clutch: When the clutch becomes worn, it can create a rattling noise.
Exactly how long a re-gas takes will depend on your vehicle, but it should take about an hour. The old refrigerant in the system has to be removed before the new can be put in but, beyond that, the technician will, at the very least, give the system the once over to make sure there is nothing obviously wrong.
Put simply, this happens because your AC system is powered by your car’s engine. When the engine is running but the AC is turned off, the AC compressor remains silent thanks to a clutch system. When you turn the AC on, the clutch engages the drive belt with the compressor and the AC kicks into life. Engaging the compressor requires some extra work from the engine and that is why you can feel, and hear, the engine work that bit harder.
The hero of the hour is the cool air from your AC system. While the AC is all about blowing cold air into the cabin to keep things comfortable, it also removes a lot of moisture from that air. The effect of this is that, when that cold, dry, AC air flows over the windscreen, it can ‘pull’ the moisture from the glass. Et Voila! Defogged windscreen!