BMW Transmission Rattle / Gear rattle / Gear lash noise

With most any lightened flywheel, everyone sees the benefits immediately due to the reduction of the rotation weight.  However, if there is a downside, it's the possibility of  a rattle sound that may be evident at idle/low rpm.  Note that this particular effect is only evident at idle with the clutch let out.  

For details on what actually causes the sound, click here.

Heavier weight transmission fluid will damp the rattle effect, but may not be suitable for cold climates or high-mileage transmissions.  The goal, then, would be to find a fluid that would have the high-temp heavy viscosity to damp the rattle but retain the low-temp thin viscosity and synchro additives to ensure all-condition smooth operation.

After experimenting with different weight transmission lubricants, we have come up with a formula that eliminates the gear rattle in the majority of test cars.

The recipe:

1 quart bottle of Redline 75W140NS Gear Oil.   ( click here to order )  The "NS" oils have no friction modifiers, thus does not increase shifting effort.  From the Redline information about the gear oil: "a GL-5 which does not contain the friction modifiers for limited-slip hypoid differentials. This makes the transmission synchronizers come to equal speeds more quickly, allowing faster shifting and much easier low-temperature shifting."

and...

1 quart bottle of Redline D4 Synthetic ATF.  ( click here to order )  "D4 ATF is designed to satisfy the improved low-temperature requirements which will be in the new Dexron IV specification, and the improved shear-stability requirements which will be part of the new Mercon specifications.  The balanced frictional characteristics provide smooth and consistent shifts for extended drain intervals.  Provides improved shifting in cold weather."

How to use:

Drain the transmission (preferably when warm so as to allow as much of the original fluid to drain).  Put in the full bottle of 75W140NS, then fill the remainder with the D4 ATF.  Note that the majority of BMW transmissions will not take 2 complete quarts, so you will be left with approximately 1/5th of the bottle of D4.

HOT CLIMATE MIX

If you live in an area that is normally hotter than average (California or any Southern or SouthWestern state) then the ambient temps might make your transmission run hotter.  In turn, this lowers the viscosity of the transmission fluid, which lessens it's ability to damp the gearbox rattle.  Such being the case, we suggest substituting Redline MT-90 for the D4 ATF.  So the mix is 1 bottle 75W140NS and the remainder of 1 bottle of MT-90.

MT-90: "MT-90 Manual Transmission Lubricant; a 75W90 GL-4 gear oil designed for use in manual transmissions and transaxles. Provides excellent protection of gears and synchronizers and its balanced slipperiness provides a perfect coefficient of friction, allowing easier shifting." ( click here to order )