My 1998 civic cx has a rapid "rattle" noise heard mostly at 40 mph or below and usually when accelerating. Also when going up my very bumpy driveway. Manual tranny. I crawled underneath and I think it is from a loose fit where the axle enters the tranny. Last July I put in a used EX tranny and a rebuilt axle assembly on the passanger side bought at Advance Auto. Could the axle be too short? What could cause loose fit? I can move the metal housing of the axle at the tranny up and down and in and out. How much play is allowed? Could the gear on the tranny be worn? Other than the noise the car runs great. Any thoughts?

Update. Friend of mine listened and thinks the inner CV joint is going.