We have a Rheem 10 Seer 9 year old Central AC. While running it cools and is quiet.
As is shuts down the most awful noise is generated outside (condensing unit). We have had it evacuated and recharged, new condenser fan motor and electrical contactor replaced. Inside Evaporation coil is not icing p, and evaporation fan is fine. No leaks and current pressures normal.
The sound starts like a really bad loose belt sound and then throws in a kind of whooping sound.
Have been told it is a valve in the compressor and the noise is from the freon equalizing on shut off.
Have also been told cannot just replace compressor must do whole condensing unit outside.

Unit is: 3 Ton Rheem 10 Seer

If this is the case, then so be it, if you have any other suggestion, they would be greatly appreciated.
Thank you,

I say the info you have is correct. The compressor is held down with pins or bolts and will have rubber gourments under them. Sometimes you can tighten just a little and might help. Do not tighten completely though. May or may not help, but just a tip

I have no idea what may be causing this noise without looking at it, it may very well be the compressor. The reason they are telling you that they must replace the whole unit is because, replacing just the compressor will cost you almost as much as the whole unit, and for warranty purposes, this makes no sense.