I experienced, what sounds like /might be, a similar issue on my truck. Symptoms included a significant loss in power and a loud "hissing" noise, similar to depressing a valve stem on an inflated tire, from inside the cab when the spooling turbos and would otherwise normally be providing boost. It turned out to be a simple fix - a hose clamp failed resulting in a leaking turbo hose. Possibly a vacuum line (I'm not a mechanic and don't have the dealer repair/work order in front of me). It took the dealer all of 10 minutes to fix (they billed warranty .1 hours) and I've had zero issues with the turbos since.