Looking at Malcolm's latest references inspired me to have a thumb through Partridge's Dict of Hist Slang.

Twang is the sex act 17th-18th century

Twanging 1609 = Excellent (and associated) Upto 1870 Twanger = anything large or fine...'go off twanging' = to go very well.
c1770 diddle = nonsense...diddle of course has lots of musical associations. Diddling is another term for mouth music. Diddling is also cheating. We won't go into 'dildo'.

Personally one off the top of my head 'Twang' = a string being plucked & 'diddle' is the vibrating sound produced, or not!