Could someone help me understand the role of flex discs? I have a 1982 300sd and have two issues I am trying to work out. One is the clunk, clunk, clunk in park with engine running. I figure from other posts is may be flex discs, center driveshaft support bearing, transmission mount or broken exhaust hanger.

The second is the clunk, clunk clunk durning downshift which is most likely a vacuum problem with the T fitting going to the modulator.

I would like to be educated on the flex discs and their role in all of this and where I can inspect them myself and what to look for.

Thanks.