Rattling

Rattles can often be not serious but they are nothing to ignore, especially on Ford steering and suspension systems. In this case, your observations suggest there could be a problem with the air conditioning compressor clutch. Depending on how they fail, they could make noise when they're engaged or only when the AC is turned off. A burning smell is often associated with a compressor clutch that is slipping.

The best approach is to have someone listen to the noise while it's occurring to see if it can be pin-pointed. I can recommend a tool for finding noises and rattles while you're driving but I don't think it would be appropriate for this problem.

The type of belt doesn't matter. Depending on how the clutch fails, typically you won't get cold air from the AC system, but it is possible for the clutch to lock up and not disengage. That will make the AC compressor run all the time. Other than the drop in fuel mileage that doesn't hurt but usually the pulley will wobble causing the belt to squeal loudly or fall off. That WILL be a problem because it will result in loss of power steering, engine overheating, and the battery running down.

After running the engine for a few minutes, carefully feel the front of the clutch. If it's much hotter than the engine, it's slipping.

Oops. As for the cost of repair, naturally you want to be sure that's what's wrong first. We don't get involved here with repair costs but I would expect a new clutch to run around $200.00 installed. My experience is more with Chrysler products. I could be way off so don't be angry with your mechanic if he estimates a lot higher. Most clutches can be replaced without replacing the entire compressor but many of them have to be removed from the engine to gain access with the special tools needed. Often that requires removing the refrigerant with special equipment, then recharging the system later. Thanks to our politicians, that is more expensive than it needs to be.

Before you base too many decisions on what I've suggested so far, you need to find out for sure what is causing the noise. Some manufacturers are going to some really weird multi-piece pulleys on their generators and they commonly become noisy at higher mileages. Those can rattle forever without causing other problems.