My experience with my 1980 300D showed that the thump thump thump was caused by the gear slipping at top of the regulator's travel because the regulator frame had cracked and misaligned the gears. The cracking is an age related problem. The newer regultor replacements shouldn't have the same problem because they'll never see another 20 years and the castings are a little better.

HTH,

someguyfromMaryland