for example fruity's 3xosc. select 'white noise' waveform on the first osc, then put the volume of the 2 other oscs to 0.

works with loads of other synths, too. or test tone generators. and then filter that noise over a length of say 2 or 4 bars. generally you'd use a rising lowpass, letting through more n more high sound as the filter rises. or a bandpass filter...


hope that helps!