Price of a new turbo on an isx15 2350.

I would try another shop. Its probably less expensive to replace the charge air cooler than have them clean it.

 

Over time a charge air cooler can get a buildup on the inside of oil vapor.
It is a dark stick layer that sticks to the inside of the cooler. It can reduce
the efficiency of the cooler. It will cause a slight power loss but not a noise like you have.
I have never seen this happen on an ISX and I work on a lot of high mileage trucks.

Did you pull the intake pipe from the air filter and check the wear?

 

Sounds like your turbo was on it's way out, passing oil into charge cooler. At least you found problem before it left you broke-down on the side of the road. $8500 must be USD? I paid $5600 CDN just for the turbo with core exchange. Installed myself (not very hard) but needed Cummings Insite to calibrate. Used a local repair shop mobile unit to meet me at home. He only had to bring laptop/Insite and took 10 min to calibrate. I also bought new actuator, but had him test old one, which passed, so wasted $1000 CDN on that. But, I keep old one in truck...just in case.