Wierd Shift
 

My 85 300SD seems to shift really nice and firm, and when it's supposed to, but for some reason there is a clanking about midway between 2nd and 3rd shift. At first I thought it was going into gear, because that's what it sounds like, but it's definitely not a gear shift because the engine doesn't change RPM. What else could cause this noise? I don't know if this is related, but the other day I accidentally put it into 3 instead of drive, and it kept bucking like crazy until I put it back into drive, then it worked fine.