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It's a weird thing happening...

'01 4.4i sport package
The other day when I opened the garage noticed the passenger rear side
lower than the driver side. As soon as I opened the trunk the air compressor
started running and leveled the vehicle but no hissing whatsoever.
That now happens every 2 days but this time is both rear side of the vehicle?
What could that be ? A sensor or air shock/bag going bag?
:dunno

On the e39 Tourings with air springs...your symptoms sound like it time to replace the air springs. If you do a quick search on that forum or google search for failed BMW air springs...you may see lots of past posts with similar issues as you've expressed here.

Have you had a chance to take a look at them? Sometimes it's obvious that its time to change them because the air bag (spring) is cracked where the rubber folds as the bag expands and contracts (see item #1 in diagram below)...and you should be able to find images on the internet of old/cracked air springs.

Below is a quote from 2010 from a discussion on the e39 forum (the Touring wagon has the same pneumatic airbag/air spring set-up)..and you can see how the rubber off the airbag on the left is cracked where it folds over near the top:

Quote:

Originally Posted by ND4SPDLSC (Post 20397670)

You can always check the condition of air springs by looking at the condition of the rubber at the base of the spring as it fold over itself. If you see cracking/dry rubber, your springs will go at some point in the near future. When exactly is difficult to pinpoint, but usually you do get warning...ie one corner or end of the car sinks overnight. If it leaks down over time, you usually have a leaking spring that will need replacement very shortly. The sooner the better as leaking springs leads to additional wear on the compressor.

http://lh5.ggpht.com/_EF1eICr7vKs/St...c/P1010032.JPG

http://www.realoem.com/bmw/diagrams/s/r/256.png

Good luck.

You might be able to spray soapy water on the air bag to confirm that it is leaking. If the air bags prove to be bad you can check out the prices on our site.

http://www.eaceuroparts.com/parts/pa...ge/31/2001/599

Air springs pictures

OK.I sprayed both air bags with soapy water and they seem not to leak at all.
The rubber of the air spring looks in good shape - no cracks or rips.
Now I don't see any rear height level change in the position of the vehicle.
Is it possible the sensor to give some false reading and then to deflate the air bags sometimes?
Here are a few pics.

The tear could be inside the roll fold and because its up against itself (rubber) the leak will not be noticeable. Don't let this fool you as the leak is very minute. If the compressor keeps having to lift the vehicle every other day the pump may burn out and those go for around $500 plus the bags which ARE bad. Change the bags and you'll be fine.
The soapy water check should be inside the back under the spare. If its all clear there it's the bags.

Just an update - yes, it turned out to be as "boramkiv" suggested the air springs had cracked over the time and were leaking to the point every night the SUV's height was dropped by 3 inches .
So, ended up purchasing and installing "Arnott" air springs from carid.com both for $180 with free shipping and Lifetime warranty.
No leaks so far!