Another good way to visualize it... buy a load of bread in a sealed plastic bag, hop in your trusty ride and find a road that goes up to 9000'. Set your bag'o bread in the seat next to you and watch what happens to it as you start climbing... it'll get bigger... and bigger... and bigger because when the bag was tied down low more air was trapped in it than there would have been up high. And of course you'll also notice the marked decrease in performance of your poor air starved engine

 

Yeah, 22Ryota, it is kinda like a screwdriver tapping against a metal object, but it has just a slight ring (or pinging) quality to it. Sounds are so damn hard to describe. I'm going to fiddle with the valve shims this weekend and see if it helps. I'll post my results. If it doesn't, I'm going to buy a real stethascope and listen carefully to the exhaust.

Actually, if someone would enlighten me... one of the possible sources I've been told is an incorrect timing belt adjustment. How exactly does that cause the engine to tap/knock. I understanding advancing ignition timing too far will cause detonation (ping). Is it the same thing?

 

Not to hijack the thread but that reminds me of a friend of mine.
She got a detached retina and went to phoenix (sea level) to get it repaired by an eye doctor. For a detached retina, they put a gas bubble in there to hold it in place while it heals. Well, when the procedure was done she and her mom went back home to Pinetop (like 7k feet). Now there is a bit of elevation distance between the two and sadly she lost vision in her eye because the gas bubble expanded, cut off blood supply and crushed the optic nerve to the eye. Of course the doctor claims he told them not to return home for that reason...which i am sure is BS and he is doing the old CYA!!! That is a very tough way to learn about physics, but she is quite a trooper and jovial about the whole thing now.

Anyway, back to engine noises.

 

My dad just had the same surgery and he lives at sea level. I'd better call him and tell him not to visit for a while.

 

Do not get on an airplane until that gas bubble is dissolved!!!
Ask the doctor how long it takes to get absorbed by the body.
She returned right after the surgery so I am sure your dad will be fine.

 

It also helps to think of it more as the air being pushed into the cylinder by the air on top of it. like a tube of toothpaste. the more weight (like standing on it) you have on top the faster it's going to go out(or in, with the air's case)