To answer your questions:
If the valve train is the same as on my 560SEL, (should be, I think), the hydraulic compensators, (fancy name for lifters), hold the rocker arm against the camshaft lobe. On my car, when it began ticking, I discovered the rocker arm had worn where it rides on the cam lobe. A couple of them actually. This makes sense, as cam lobes are hardened in the wear area, causing the rocker arms to wear out first. So, I replaced only a few rocker arms and was back to a quiet valve train. You need a special tool to remove the rocker arms, (on mine anyway). I have heard the rocker arms wearing out will eventually ruin the cam lobe, so you need to get after this repair soon.
If the manual for your model is available on CD, buy it! The one I bought for mine is great! Full of good info. Try Fastlane or partshop above. Seems expensive, but after one or two repairs, it has more than paid for itself!
1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)