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!!
