412 EP Auto Manual Governor Check
The popping noise tends to be louder on the 3D series than on the 3B.
The noise is also more likely to occur on reselecting Auto after gently winding up to 70% N1 in Manual to check normal response then closing the throttle prior to reselecting Auto.
The popping can be minimised by reselecting Auto 2% - 3% N1 above the normal Idle setting - i.e. around 64% N1 decreasing (normal Idle 61% N1 + or - 1%)
Trog