ML 320 loud Hissing sound

[gfdent]

2001 ML 320 driven home last night and ABS light came on, followed by a loud air hissing noise under the hood somewhere by the left strut tower. Not by the firewall or the front of the car, this sound is coming somewhere around some components in the vicinity of strut tower and engine. Turned off the engine and removed the key but the sound continues. check A/C lines everything ok. Coolant system is fine. Removed the postive battery terminal and the noise stopped! Put it back on and it continued, so I left the terminal off. It doesn't sound like a motor turning, it sounds like alot of air. Would vacuum continue after engine is off and key removed? can someone give me some insight to what this could be?