808nation

I press the brake down and press the start button, as the engine is starting up I hear a clunking sound for a couple of seconds. This happens occasionally and not every time, anyone have similar experiences? I took the truck in to Mercedes and they could not verify the noise, they also checked the engine and everything was good to go.

chassis

Need more information. Where is the noise coming from? Exactly when does it happen? How long does it last? Can you force the noise to come and go, or to change intensity?

amusa

Clunking noise? Not a tap tap or clicking noise?

It can be anything... Need more information.
What motor oil velocity and weight your using?

808nation

Noise coming from engine bay. I press the brake and press the start button and as soon as the engine starts I hear a clunking noise and lasts for only a couple of seconds. I cannot duplicate the noise as it happens once in a while, and that is why Mercedes dealership is unable to find the issue. Noise does not change in intensity.

Question: How hard to I need to press down on the brake pad to start the engine, I floor it until it can't go any further and then press and release the start button....maybe I'm releasing my foot off the brake pad too soon or not pressing down hard enough?????!!!!!

808nation

I use whatever Mercedes puts in during yearly service. Definitely not a tap tap or clicking noise. To me is sounds more like clunking noise, like them old cars and clunk

amusa

The way how you describe it sounds like a rod knock. Rod or main bearing is loose would be my guest. You said you don't hear it all the time on start up. I question it and you have a 2017. That is very odd.

I just thought about the drive shaft. I would inspect the Drive Shaft Flex Disc first.

SUV John

When starting, a light press is all that's needed- no more than if you were at a traffic light.

I've had noises similar in other cars where the issue has been a component such as a heat shield that vibrates/rattles when the torque of the starter motor is starting the engine. I wouldn't suspect something like the drive shaft that isn't moving when the car is stationary.

Try listening under the hood closest to where you suspect the noise is coming from while having someone start the car for you. I think the good news is that if the problem isn't constant then it won't affect driving at this time.