The DISA valve is the black box on the side of the intake manifold, and it adjusts the length of the intake so that there is more torque at low revs. The shaft of the valve breaks at its base, and then the flap falls off: the pieces can get into the engine and cause major damage there. Its membrane can tear separately as well. The symptoms are juddering on acceleration and a noticeable loss of power, and the valve can also hiss loudly after the engine is switched off. The seal of the intake connection is cast into the DISA, and as it shrinks it lets false air in. Its condition can be checked with the valve removed. The flap has to turn against resistance, spring back when released, and have no play. With the vent hole at the bottom of the round valve held closed, a flap moved by hand may only spring open to 45 degrees and then has to stop. With an aftermarket DISA the experience is that the trouble is back after a year, so a repair kit or a reconditioned factory part is the better choice.