Big end shells are the bearings surrounding the crank shaft at the bottom of the engine. On some vehicles you can replace them by dropping the sump and gaining access through the bottom. It all depends on the condition of the crank, this may require replacing or machining if scorred. Has she run short of oil? or gone extended periods without oil changes??
If it is the big end, it will not be cheap. I suggest a good mechanic take a look, I could tell you but its very difficult to diagnose over the internet as people's perception of knock can be totally different... A goosed big end is characterised by a knock at idle which changes into a rumble as revs increase - especially under load.
here is another possible sitch (situation) I had a 300 SDL many years ago with the same issue, I don't know what your knock sounds like but mine was not a tick it was a knock that did get louder coming from the engine. Well it was # 6 cylinder that the rings were shot and the piston was slapping knocking against the sleeve. Long and short I did new sleeves in all six cyls, crank redone with new pistons and bearings, I reused the rods, put it all back together and it was perfect. So you might need to inspect the cylinders to determine this.
Just my thoughts...Good luck though I hope it's something much less drastic.
Could be a injector / nozzle: injection pressure is to low compared to other nozzles, fuel is injected but not completly burned in time so provockes this knocking when burning to late. At higher revs the pressure is more in line with other nozzels so no more knocking.
garage rang me to say that shards of metal were spitting out on tick over from around a large fan and pulley area at the top front area so he thinking a bearing may of collapsed, but its not the engine thank god .