Hi Ormondstreet,
First of all (to help us diagnose your problem), which engine do you have? How old is it? How many miles has it done and how often has it had oil changes etc?
If it does boil down to your big end bearings you will have to strip down your engine in order to change them, which is annoying considering that you can get to them by simply draining all your oil and removing the sump, but you need to knock the pistons upwards (which will be stopped by the head) in order for the con rod to clear the crank shaft so that you can change the bearings. Unfortunately I don't know what kind of sound they would make, but I can imagine it being like a washing machine drum out of balance with a heavy load.
The fixing process involves:
1) Removal of rocker cover (usually three bolts / nuts)
2) Removal of rocker assembly (eight bolts)
3) Injectors out
4) Wiring (glow plugs) removed
5) Headbolts out
6) Head off
7) Sump off
And then you can undo the big end caps and knock the pistons up and out of the top of the block so that you can get to the bearings. Ofcourse, at this stage you would be stupid not to replace your piston rings and deglaze the cylinders. It's not that big a job: deglazing tool is about £15 and fits in your drill, piston rings are about £6 per set of three, and your engine will be like new again. You'll need the best part of a three day weekend to get it all in though (unless you're experienced, and you'd probably get it done in a day with all the right tools etc.)
Obviously I can't provide you with any more specific information until I know what engine you have, but it might be something much much simpler that involves next to nothing to fix!
-Pos