Vibration/noise in 98 Nubira

[revken]

hi everyone. I have a 98 Nubira manual sedan with several issues that have all arisen in the last 10,000 kms. I've replaced shocks and a drive shaft. I'm pretty sure the left engine mount is kaput, and as soon as I work out where to get one from I'll get one. The main problem I'm having at present is this. There is a noisy vibration in the car in third gear and fifth gear. It is most noisy in fifth. It disappears when the clutch is disengaged. I'm pretty sure the clutch is on the way, it can slip when accelerating hard, but I'm not sure if the issue is the clutch or the gear box or something else. the fact that it is different in different gears leads me to believe it may be gearbox, which has me quite concerned naturally. I don't really want to spend a huge amount of money on this thing, it's only supposed to be a stop gap. I'm hoping someone may have experienced this before and might be able to give me some clues as to what might be going on. Thanks.
Rev Ken in Adelaide, South Australia.