2ltr 16v 5 Speed Manual - Grand Vitara 2003
There seems to be a slight rumble from the gear box (maybe?) The occurs in all gears bar fourth when accelerating lightly. Any ideas as I'm at a loss. Suzuki say the gear boxes are 'bullet proof'! Gear box oil has been changed regularly. Vehicle has only done 45000 miles!

"Bullet proof" is a relative term - you may just be the unlucky "one failure in a million" guy.

I remember reading somewhere that all the gears except fourth are "indirect" and involve use of the "layshaft", so a noise which is everywhere except fourth is a layshaft bearing.  This wasn't specific to a Suzuki, so offered with a caution.

Mine was noisy in every gear but fourth turned out to be the rear bearing on the intermediate shaft. Fifth was the noisiest. I pulled the rear of the tranny off replaced the bearing been good since. If you're losing a bearing a look at the oil will make it quite obvious. Mine was really silver before I fixed it. It was a pretty easy fix The tranny even stayed in place I just had to pull the rearmost section off of it. However there is another bearing on the other end of that shaft. Good luck

Thank all for your help. Much appreciated. I will check the oil this weekend and update. Problem sounds similar. Lets hope it's not going to be too much of a drama!