When you first hear the Getrag rattle its a little unnerving. The Getrag rattle is the sound the Getrag transmissions (particularly 5 speed’s like those found in E30’s, BMW’s and GM’s) makes when it is in Neutral and the clutch pedal is not engaged.
The sound completely goes away when either the clutch pedal is engaged or the car is in gear.
Some would describe the sound like a bag of marbles, gear chatter, or similar metal rattling.
The sound is not a fault within the transmission, rather what you are hearing is the unengaged gears vibrating against their neighbors.
If you have ever heard a large 18 wheeler idle in neutral you would have likely heard a loud metal rattling sound as well. Again its not because the truck is broken, its because of the play in the gears when not engaged. The sound is louder in an 18 wheeler because the gears are bigger.
As the transmission accumulates miles the sound may get louder as the gears and bearing clearances inside become ever so slightly larger. It is not indicative of the transmission failing.
Never the less its annoying and sometimes embarrassing.
The noise can be significantly effected depending on the transmission oil you use. The thicker the oil, the harder the gears and bearings have to work to bounce around.
The solution is to mix one part 75w80 (or similar) and one part heavy weight oil like 75w-140 or 85w-140.
Using overall thicker oil will increase shifting effort in cold winter weather until the transmission is warmed up so keep that in mind.
One critically important step is to use GL-4 oil!
All the oils I could find in NA were GL-5 “compatible” for applications calling for GL-4 or a fixed weight. Although there may be some compatibility, the composition of the grades is fundamentally different and really werent designed to be backwards compatible. Technical explanation below why it is not truly backwards compatible:
I was able to find this oil from Europe which is exclusively rated gl-4 and for manual transmissions:
Which I was able to order from amazon.de from a seller who ships internationally:
I got a 5L instead of a 4L, nice 1L bonus!
Find below an easy way to refill your transmission fluid in your E30: