A-833 Rapping

Sounds to me like a chipped tooth on the input's drive gear, or possibly on the cluster gear that mates to the input.

It's less noticeable in 4th because it has no load going through the cluster gear.

You'll likely need both an input and a cluster gear, but you might get lucky and have the cluster gear or the input still be in good shape, depending on which is bad.
They may both be.

Mark.