[Echoic: cf. TAT v.1) = RAT-TAT.

1

1786.  Mme. D’Arblay, Diary, 17 July. A tat-tat at my door followed, and a lady entered.

2

1902.  H. S. Power, Tale of a Piccaninny, 5. in Australian Poems & Ballads, 14.

        He beat tat-tat on a broken shield
  With two stiff yam sticks old.
The big snakes stole from the still lagoon,
  To list to the tale he told.

3