2002 C240 72k Miles

New to stable. Noticed that while backing up and you give it gas say in an open park car makes a whine, seems to come from diff. But there's barely to none same noise while you are backing up slowly with foot on brake, neither is there any noise at all while in drive. So far all gears are shifting just fine (If I should mention a 2000 BMW Z3 85k miles, makes the same noise also but only while backing up, have a 97 Camry 182k miles that's as quiet as a church mouse while reversing.) Have searched but what I am seeing are issues with clunky sounds, whining while in drive but nothing while reversing. I plan on doing tranny and diff fluids. I plan on doing tranny and diff fluids. Is the diff ok, what should I look for? Any diagnosis tips?

 

Mine does it too...manual tranny..hasn't bothered me enough to do anything about it

 

I think I finally found the answers, unless there's something else someone can educate us on, I am all ears.

Apparently it has to do with design of reverse gears - they are not helical but straight cut, aka spurs.
Here is a straight explanation, and an expounded one here complete with a video of a very skilled race driver.

 

What you found is probably correct. I have a 4 speed manual trans in my 72 Camaro. Yes its loud, but kind of in a good way.