Defender td5 grinding/vibrating noise

Hi all, im new to this forum so bare with me.
I have a Defender 90 td5 (2003) and last week it has started making a grinding/vibrating noise. at first it was just at start up when cold but now it is happening when driving, when accelerating upto 30-40mph,the vibration seems to come through the pedals but i dont know how the hell to describe it and its very hard to pinpoint the grinding noise, however it isnt constant, just short sharp bursts sometimes loud sometimes not so loud,(it also happens when im slowing down whilst in gear, most noticably in 3rd gear Its my first landy and il admit im no mechanic, but i had someone have a look and was told it was either clutch or gearbox but it was 2 soon to determine the exact problem and was told to let it develop? Can anyone help please coz i just want it sorted asap wotever the cost, have bought some molyslip to give it a try on the gearbox but not sure if it will help. Please help!!

when was the last time you checked you gearbox, transfer box and axle oil levels? or even changed the oil in them? I would start here.

I have bought some molyslip gearbox additive and was thinking of changing gearbox oil? clutch oil level is fine, but how do you get to gearbox to do oil change, like i said its my first landy but i only bought it in febuary, and it had a full service done before i picked it up. does th oils get changed wen servicd?