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.
Cheers