Knocking noises; vibrations in the handlebar; oil sipping at valve lid gasket

[Cormanus]

I'm not a mechanic and don't pretend to know what is causing the noise in your engine or whether it's even a problem, but I have a recent experience to share. I own a boat with an ancient diesel engine. It was serviced recently by a young mechanic who took it upon himself to tighten up the valves which were all loose — his words not mine. It made a truly remarkable difference to the noise the engine makes. I hardly recognised it when I first started it. It is much quieter and a notable knock that was there has disappeared.

I'm sure max, pekingduck, peterbaron, Lord Popgun or others will be able to tell you whether that is a possible explanation.