Hi Dave. Welcome to the forums! Two minutes after you post your question, I am here to assist!
What you might have here is an air-oil separator problem. This device sucks air out of the crankcase and then funnels it back into the engine intake so that fumes from inside the engine can be burned again, instead of being released to the atmosphere. It's an emissions device.
I have a complete article on replacing it right here:
Pelican Technical Article: Boxster Air / Oil Separator Replacement - 986 / 987 Take a look at the article, and it should give you an idea of what is involved.
As for the replacement costs, 10 hours is completely ridiculous in my opinion. I'm thinking that your shop has never done this before, and is probably quoting you high to cover their butt. As for the 02 sensors, it's not so much that the O2 sensors get messed up, but the catalytic converters might too. What you need to do is to replace the air-oil separator unit and then run the car for several hundred miles to burn all of the crap out that may have entered the exhaust. Then you check to make sure that everything is working properly.
As for the loud "ringing noise", I have never heard this noise myself, but I am told that it's caused by air getting sucked into the engine through some of the seals. When the air-oil separator fails, it takes the intake manifold vacuum (which is quite powerful - it is used to power the power brakes for example), and starts sucking on the internals of the engine. This causes air to want to seep into the engine through various seals (which may be old and may have some areas where air can leak by). This intake vacuum on the engine case can cause this high pitched "whistle", as I have heard it most commonly described.
Let me know if you have any more questions!
-Wayne