Hi, I have a '78 280SLC with the same engine. It may pay to check the valve clearances. It sounds as though as the engine is heating up, things are expanding and clearance is getting smaller. Oddly, my one has absolutely no valve noise on cold statrup, but after it is warm I get a minor ticking.. The (cold engine) clearances for the M110 engine are 0.10mm for the intake and 0.25mm for the exhaust. Give it a shot, it can't hurt.