There are so many possibilities…
Where does the noise seem to be coming from? Engine compartment? Under the aisle floor? Differential? One corner?
When you take it to your Sprinter dealer for service, get a technician to ride along with you, so you can point out the noise to him. Dollars to donuts, even though you can spot the noise every time you hear it, the technician won't pick it up the one time he hears it, without someone pointing it out to him.
Engineering: Finding complex solutions to simple problems you didn't even know you had.