You can sometimes hear the tappets rattling at the top of the engine due to excessive clearance caused by wear, but some particularly old design engines rattle anyway when the clearances are correct.
However, the real danger is the valve clearance closing due to valve seat wear, which you cannot hear, and only becomes noticable when performance drops off and the engine can be dificult to start.
By then its too late and the sealing faces on the valves will be burnt and no longer sealing properly, requiring a major strip down and rebuild.
So it’s important to check valve clearances regularly according to your workshop manual, which involves removing the top engine cover and checking with a feeler gauge. It’s an expensive job to have done due to the skill and time it takes, as many engines require re-shimming rather than adjustment, but you can do it yourself with care.
British Superbike School 