How certain are you that the compressor is cycling on and off at highway speed? The hiss and moan you are hearing is the expansion valve admitting more or less liquid refrigerant into the evaporator. It's a typical sound on a 124 which is a bit low on freon.

The rattling compressor means you are likely going to buy a compressor pretty soon. Try having an a/c shop add some refrigerant oil to the system to quiet it down. They can do this without discharging the system. It may buy you some time. Not running the a/c until you get it serviced might be a good idea too. (Yeah, I know how hot it is in Ft. Worth...)

- JimY