Pierre,
Sounds like you have a major vacuum leak. Check all the vacuum lines that lead away from the vacuum pump on the front of your engine.
Since it's a big and loud sound, it could be one of the larger vac lines that go to the brake booster.
Not having full vacuum in your brake booster could be unsafe.
If it's just a slow leak off of the vacuum in the booster canister when you shut the engine off, that's pretty normal, but it sounds like you have something larger going on.
As far as getting better mileage, do you have a Scan Gauge II (or similar) so you can regulate the % of engine LOAD.
Perhaps you may not understand the MB engine management systems.
You can be chugging along at ~<2400 RPMS in top gear (which transmission do you have...the NAG-1 5 speed auto like in the USA or something different) which is an overdrive gear, and be using the maximum amount of fuel and packing in the maximum amount of turbocharger boost.......and you will never know it.
A simple manual downshift, into a more powerful gear, will bring your RPMs up into the range where your Sprinter makes it's best and most efficient power and your fuel consumption and boost rates will drop significantly.
The Scan Gauge II (or similar) helps a lot as it will tell you what your current % engine LOAD is, and you can make conscious decisions on things you can do to reduce the % LOD....which can reduce your fuel consumption.
Manually selected lower gear ratios, higher RPMS DO NOT result in more fuel usage in a Sprinter with the MB engine management systems as the fueling rate and turbo boost rates are completely under the control of the engine management systems....NOT under the control of the driver.
The driver simply has the ability to tell the engine management systems that you want More power.....maintain your current power.... or less power!
More power means the fueling rate goes up...often to the max, in a heavily loaded (and wind loaded) RV.
Maintaining your power means the engine management system will adjust everything to keep you going the same speed....and... if anything changes (like more wind or an uphill grade) it will add more fuel and boost to maintain the current speed.....UP TO the absolute MAX.
Less power tells the engine management systems to reduce the fuel flow to 0.000 GPH/LPH.
When the %LOD on a Scan Gauge II reads 0%..... there is no fuel going into the Sprinters engine at all. You are just coasting along.
So, in summary, the driver has no way of knowing what the % LOD currently is, and therefore has no way to really control the amount of fuel used. With the SGII you can see, in real time, what your engine management systems are doing, and make changes to improve your fuel consumption figures.
Roger