Today I went for drive and I noticed the rear left side was making a loud hissing noise and it was sitting lower than normal any badly have any idea why this is happening???

 

Most likely cause is that the flexible fabric/rubber rolling diaphragm in the left rear suspension strut has failed, requiring strut replacement.

You can purchase a rebuilt strut with a lifetime replacement warranty from Arnott. Installation is within the capabilities of a reasonably experienced DIY mechanic. There are numerous videos on YouTube.

More information is available here https://w220.ee/Airmatic

 

This happened to my 2001 S500 a few months ago, but on the rear passenger side. You'll need to replace the strut - if you get up inside the old strut between the outer and inner sections you can dab a little soapy water on the rubber bellows and you'll see where the air is leaking out.

Best price for a reman Arnott strut I found was from Rock Auto, around $250 plus $50 core charge.

It took me about 2 hours to replace with minimal tools.