In analogue synthesis, there is very often a controller referred to as VCA BIAS/VCA Control. This can come in the form of a switch, a dial or a fader and is present on a number of Behringer synthesizers. The VCA refers to the initial volume of the synths output before any modulation or filtering is applied to the signal, therefore if this is set at a high level the synth is likely to self-oscillate. If you are using a MIDI input source (or the internal keyboard if applicable to your synth) you will want to ensure this is turned completely down in order for your envelope settings to take effect.