Turbo service of my 2008 nissan navara was done Solenoid valve changed (I have varex exhaust on my vehicle) While my vehicle is going uphill to change gears, when the speed is increasing at 80-100 km, there is a sound fluctuation in the form of turbo vuuuuvv vuuuuuuvvv vuuuuuuvvvv. The master that I had it serviced said it was due to the exhaust. What causes sound fluctuation?