That farting sound is actually from the pressure relief valve as
mentioned by some folks earlier on. This "Farting" sound is
actually there to ensure that the air presure in the air tanks are
kept with the safe operating range and as for the air dryer, its
purpose as it's mane suggested is to remove the moisture from the
atmospheric air before the dried air goes into the vehicular
penumatic system.
The button under the drivr seat at the floor board is known as the
exhaust brake. This component serves two role. Firstly, it allows
the driver to slow down the vehicle while going dwon steep slopes
and secondly, whe the engine is running at idling speed, when you
are to depress this button, it actually triggers a plungger at the
fuel injection pump located in the engine bay which will cut off
fuel supply to the injectors thus cutting off combustion.
But for the MAN LAERC 16.284 tonners, the switch is only
operarional at speed in excess of 15km/h and it serves only
function whichj is to slow down the vehicle.
But for all the other older penumatic vehicles lik the U1300L
Unimog and the 90wm 17 Iveco 3 tonner or the 14.240 MAN 5 tonner,
the switch serves dual function as mentioned above.
My 2 cents worth.
Cheers!! 
