My GL350 developed a fairly annoying resonance vibration that I could hear coming from the rear of the truck. It was most noticeable at idle, a loud humming sound, clearly resonating at idle speed. It was very definitely RPM related - it was loud within a certain RPM band range and then went away entirely as engine RPMs built. it was also louder in reverse than in drive.

It was pretty easy near to narrow it down to the exhaust. I could feel the right rear muffler vibrating and holding on to it tightly stopped the noise.

The noise started after the dealer dropped the DEF tank to fix the heating unit inside. I assume they removed the exhaust during this procedure and when they reassembled, they introduced this resonant vibration. The dealer had previously replaced the engine mounts to try to make this noise go away, and while it was reduced for a while after they did this, it came back with a vengeance a few months later.

The actual exhaust pipes are several tubes held together by clamps. The first clamp attaches the main exhaust pipe to the diesel particulate filter. Then there are two clamps after the Y that separates the main exhaust pipe into two pipes headed to the left and right mufflers.The exhaust system hangs from your truck body via 4 rubber exhaust hangers. There are also 4 cylindrical resonation dampers, one attached to each muffler and two to the exhaust pipes, all attached near your rubber exhaust hangers. These are small heavy metal cylinders coated with rubber and are about 2 inches in diameter and 3 inches long. Last there is a cross brace just before the mufflers that joins the two sides of the Y together.

​​Replacing the rubber exhaust hangers did not solve the problem.

What did solve it was removing all four dampeners, and then completely loosening and removing all three exhaust clamps and the cross brace. Then I ran the truck for a few minutes to let everything move around and loosen up. As soon as the clamps were loosened, the vibration and noise went away.

Next I reinstalled everything, one component at a time, starting at the engine and moving backwards. I used anti-seize lubricant on all of the bolts, and blue loctite on the vibration dampeners.

I started the truck before re-attaching each component, so I would know if anything I did reintroduced the vibration.

When I finished re-bolting everything back together, the vibration was gone.

If if anyone else has a phantom resonating vibration that is RPM sensitive coming from the back end of your truck, perhaps this will help you figure it out...

 

I was wondering if this solved the problem long term. It's been almost two years.

 

No this poor OP got taken for a ride by the dealer and had to fix their induced problem himself.