Hi, I have a 2012 GL 450. Everything seems to be fine but when I drive every once in a while on the right front side, I hear a hissing noise. It does it for a few seconds and then it stops and then every other once in a while it does it is this normal or is there something going on?

If it’s a hissing noise coming from the right front (us passenger side) fender folllowed by a whoosh after opening a door that is normal. Your valve block is venting excess pressure overboard through the compressor intake pipe.

if it’s a a hissing noise that comes and goes independently of opening a door and leads to that corner being noticeably lower than the others, that is not normal and could be a leak in an air spring.

the overnight sag limit is 10cm.

What! That is like four inches!

Basically bumpstops.

Oh I thought you meant the diagnostic limit.

How about if both front sides are low. Like maximum low? My car was parked over several days and one morning I come out and see the front tires are just about at the top of the wheel opening. Started the SUV up and it immediately rose to normal. It has stayed at normal after a little driving and again parked for several days.

Sag limit is 10 cm overnight. That is a very generous limit.

if you have visible settling overnight, i say you’ve got a leak.
go hose both front bags down with leak detector spray or as Eric would say, soapy wooder.
if you continue this kind of behavior you will burn out the compressor in no time.

Think is, it only happened once. What would cause that? Seems if it was a leak, it would happen continuously.

and all depends on how you parked or the angle of the dangle that uncovers a hole in the bag, which then leaks down to it doesn’t leak anymore or don’t leak at all.

Thanks. It was parked where I normally do and no change over the past several days. But, I'll keep an eye on it. Doesn't seem like using a leak detector now would help. I'm measuring the gap between the top of the tire and the wheel well in the meantime.

Also, the fact that it was down equally on both sides suggests it wouldn't be a leak in the boot, right? I don't know how the air lines run, but it only affected the r/l front, too.

if you have a leak in one bag, the other side will try to auto level itself. that could be one reason for a car diving down in the front, or rear - another there is that you have a leak in both bags on one axle.

the third reason, although less likely is that you have a leak in the valve block itself.

Any possibility it could be battery related? Although the batteries (main & aux) are fairly new, the SUV has gone from being used regularly to only occasionally. I do have a dash cam running in parking mode that is triggered often from passing cars. I know the battery can cause all sorts of weird events. No low battery or charging warnings, though and car starts normally.

The airmatic suspension will check and if necessary; auto level every time a door opens. If you have a door sensor that is malfunctioning that could give erroneous commands to the airmatic to go level itself and thus deplete air pressure in the central reservoir.

but I don’t see how it could be battery related.

Ah-ha! That could be it. My wife didn't close her door fully and maybe some movement caused the deflate. Anyway, I'll be paying close attention for any changes. Thanks again.

Turns out the passenger side shock had a leaking seal. Decioded to replace both front shocks and airbags. $1300 parts & labor.

What brand parts did you decide to go with ?

My mechanic bought them and i didn't ask because he usually uses OEM unless very overpriced and he guarantees his work. Based on the overall price, he didn't use OEM, but high quality aftermarket. Looking at them installed i couldn't see any obvious differences. I did see a label. I'll edit the post if i see a brand name.