Volvo D13 turbo question

What's the symptoms of bad turbo? Truck is driving well. There was some codes for turbo on it. Could it cause big fuel consumption? While truck idling I can hear that turbo starting to whistling then it stops and start again and do it repeatedly. Fuel consumption even while idling is 1.5 gph. And on the highway around 5.5 mpg which is pretty high for that engine. May turbo cause such problems? Or turbo actuator? Or EPG (but engine break works perfectly).
Thank you

 

I never had problems with turbo before, so I don't know how to check if that's a time to replace it cause I won't do it on the road

 

Code I have spn 102 fmi 18 which means low intake manifold pressure I think

 

Or could it be exhaust pipe clogged?

 

Still very bad fuel mileage, average around 5.5 mpg. Engine D13. Any ideas what may cause that?