Edit: The noise i was hearing is not suspension related at all: check my post’s reply to see an update.

I bought this 2007 Mercedes E550 4MATIC with 103k miles 2 months ago.

After getting my tires changed, I parked the car for 16 days. When I went to drive it again, the rear-right corner was completely tucked into the wheel well while the other corners looked normal.

When I started the car, the rear-right raised somewhat but still sat lower than the rest. I also noticed a consistent high pitch hissing/squeal from the rear, much louder on the right side. Here is a video: While underneath the car, it kind of sounded like it coming from the rear-right expansion tank area, but it’s hard to tell with all the engine/exhaust noise.

I drove it around and when on level ground, the rear-right came back up, and the rear-left lowered itself more than the right (I tried replicating the next day and the right stayed lower this time). The compressor keeps cycling in short bursts, especially on uneven surfaces. It seems like the system is continuously trying to correct height. It doesn’t just turn on to pressurize the system, I hear it releasing air too. It even cycles while driving slowly down a straight road. It stops cycling after the car is no longer moving.

So far I have:

• Checked for leaks with soapy water around the rear bags/lines/fittings (no obvious bubbles). I need to go back with better spray bottle, because I couldn’t make heavy suds.
• Checked rear sensor. The metal ***** are corroded and the plastic linkage requires pressure to move it. I plan on applying some lubricant to see if it helps at all.
• Used a Foxwell 710 scan tool to manually test the compressor (passed), raised/lowered both rear corners, and turned on the component Y54/1y1 (Additional volume valve’s) on both sides, which made a buzzing sound.

What’s strange is:

• During manual scan tool tests, both rear corners raise/lower quickly and I did not hear the hissing/squeal at all.
• The hissing only happens when the engine itself is running
• The noise stops immediately when the engine is turned off
• I get no suspension related codes during scans. Before and after clearing codes and going on short drives. No dash lights, error messages, etc.

The only recent work besides the tires was a rear differential rebuild about a month ago from a reputable shop in Portland, OR. They did an inspection beforehand and said all suspension looked good, other than the front-right (it was slow to raise compared to the rest, but otherwise all good).

Any suggestions for next steps for diagnosis? Any input would be appreciated. I have done a lot of searching around, any “hissing” others seem to be referring to is the “pssh” noise the compressor makes when releasing air. Mine is nothing like that, it’s more like a squeal, and in the rear of the car.

 

I have an update.. After using a mechanic stethoscope, I tracked squealing noise to the gas tank, and the hose clamps on that hose. Then I found similar noises on the EVAP vacuum lines next to the intake manifold.

So, it turns out I probably have a leak on the rear-right air shock, and some sort of vacuum leak (and/or something fuel related).

Get a mechanics stethoscope y’all! Can’t believe I waited this long to get one.