I've had my 2014 E350 sport for 2 months now and tried the sport mode for the first time. Shortly after switching to S mode, while stopped at a long light with eco on, there was a strange noise that sounded like an on and off pulsating revving noise, that repeated until I stepped on the gas to go.

Any clues as to what this could be?

I think, ECO mode is not recommended in when you are in Sport mode. Please switch off the Eco mode.

I don't recollect seeing that statement anywhere in the manual. BTW the ECO button controls only the engine stop-start function. It should not have anything to do with driving in "sport mode" (which affects mainly the transmission).
Does the sound happen while the engine is "on" or "off"?

Normal... I have it in my 63 and had it in both my 2014 e550's

It's the trans pump likely circulating transmission fluid

Thank you all for your responses.

I didn't know that Karora, Information about E or S mode is not in the written manual it say's to check the onboard manual which I will.

Pamiboy, I think eco was yellow and turned to green about that same time. I got to thinking that possibly this was a glitch with the eco trying to activate??

PeterUbers, It could be but, I've never heard it before - seems as though a noise when the trans pump is circulating fluid would have been heard before.

Then it's not normal, I don't know lol... Ask your service tech

Post a video of the noise... I'm sure that will help tremendously

Is the engine OFF (Eco activated) when you hear this noise? You didn't specify

And when you step on the gas the engine turns on and the car drives normal?

Thank you again for responding PeterUbers, it's very nice of you. Stupidly, I don't know if the eco was activated or not at that moment because the "ECO" on the instrument panel was yellow and I wasn't watching to see when it turned green. Unfortunately I had also switched to S Mode not paying attention to whether or not the ECO was yellow or green. The noise might even have occurred when it did turn green and activated. The noise did go away when I stepped on the gas and the car drove normal. I didn't hear the noise again. I read where the eco only comes on in an S Class when in E mode but can't find anything about that for the E Class, even in the onboard owners manual.

Re: "Post a video of the noise... I'm sure that will help tremendously" I wouldn't have a clue how make a video of the noise, lol.

So you don't know if the engine was on or off? Happy to help

I would imagine if you heard a new noise it was because the engine was off and you were more aware of a new sound

"So you don't know if the engine was on or off?" No, unfortunately not. I'll have to see if it happens again and, if so, take it in to the dealer. I sincerely appreciate your help though.