Hello from Atlanta---I had a similar clanging last year on my cat equiped
'81 air cooled.  I spend lots of time around and under my newly rebuilt and
reinstalled 2.0 trying in vain to find the clanging.  Finally did,
though!!!  There was/is a baffle of sorts made out of sheet metal that
mounts to the rear of the van and fits above the muffler, and is about the
size of the muffler.  I could never see this metal section from below as
the muffler obscured it.  I finally noticed it through the louvers behind
the rear bumper....  The clanging?   The installer the week before chose
not to reinstall one of the sheet metal screws that hold this metal baffle
in place, and left one of the other screws a bit loose.  This was the
source of my clanging!!!  At different revs the metal would vibrate against
the muffler and cat.
I fixed the problem when I spent one hour finally getting the missing screw
installed---for me it was in an almost inaccessible place with the muffler
installed.....

I hope this helps!

Regards,

Michael and Christina Nichols
2--'81 air cooleds

PS---Over the week following my clanging repair I  found and tightened many
screws, gas line clamps(!!!) and starter connections that were reinstalled
only hand tight, thanks to the shop that did my rebuild here in Atlanta.....