Depends on the model.
Some of the 2010 STs had actuator issues. You'd hear a regular thump-thump-thump coming from somewhere (and it is coming from the actuator).
There's a small round thing sitting on the upper part of the turbocharger (this is the actuator). It has a small black hose coming off the top. That's the vacuum line. It connects to a couple of other hoses - any one of those could make a vacuum leak and cause trouble. Let the car idle and watch the little metal arm that comes out below the actuator - it should move in and out a little bit as the ECU adjusts the vanes. If this movement isn't there, check the tubing. If the tubing is all ok (and you can detect suction in it, remove it and put your finger on the end while the engine is running) then the vanes may be stuck/sticking.
If the vanes are stuck it's a sign the turbo is either faulty or wasn't looked after particularly well - and I'm thinking bad air filters letting in dirt more than any other cause.
Personally I'd probably rebuild the turbo if the vanes were stuck. You could try spraying some lubricant into the turbo itself but I'm not sure that's the best way to deal with it.
