When stopped at a light and transmission in drive, occasionally feels as if someone lightly bumps you from behind, Very subtle. Does not happen when car is in Park. 2003 G35 auto trans sedan. 170K miles.

 

Check your differential bushings. Very common wear point, your subframe will have a tear beneath where the rear of the diff mounts.

 

Thanks. Thought it might be transmission related since it only happens when car is in "drive".

 

Might also be a worn thrust bearing in the differential causing excess play. With the vehicle on jackstands and the transmission in neutral grab the driveshaft right at the differential and turn it back and forth and see how much play it has.

If you have worn diff bushing AND too much play in the diff gears it's multiplicatively bad and feels like axle wrap. This feels exactly like someone just bumped into your car from behind, you'll notice it in that split second between when you let off the brakes and when you actually start rolling forward.

Basically what happens is you come to a stop and the transmission disengages ouput shaft rotational force, but the diff unloads and physically ROTATES FORWARD due to play in the diff bushing as the vehicle settles to a stop, this also opens up play in the diff gears as it creates a separation in the ring and pinion gears, then when you let off the brakes the differential rotates backwards AND the ring/pinion gears slap back together as torque is applied from the transmission.