Yes, RAMBLING is a Scrabble word!

Letter values in Scrabble US: R 1, A 1, M 3, B 3, L 1, I 1, N 1, G 2.

Is it a valid word?

RAMBLING is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

continue talking or writing in a desultory manner
move about aimlessly or without any destination, often in search of food or employment

spreading out in different directions
(of e.g. speech and writing) tending to depart from the main point or cover a wide range of subjects
of a path e.g.

Also: ramble on.