Originally Posted by aussiem3
It is audible at 60km/h cruising in 5th or 6th gear
That's not drivetrain slop then. You don't have any slop when drivetrain is under load. You're probably talking about a 'whine', either from the tranny or differential. Most trannies and differentials are going to whine, since they're all churning gears inside, but shouldn't be audible inside the car with stock shifter/insulation. Start by checking fluid levels. The diff is known to clunk due to improper fluid, but that's only when turning sharply.
If you feel it's not normal, I'd first try to meet with other forum members to confirm your suspicion before heading to a dealer. It's hard to even give you an idea what it is over the internet, without any sound clips. Good luck.