Here are some possibilities to investigate:
* Loose or worn rocker arm: This is a much more common cause of a knocking sound that can sometimes be intermittent or change with engine speed. A loose rocker arm can create a tapping or knocking sound that might disappear under certain conditions.
* Exhaust manifold leak: A leak in an exhaust manifold can create a knocking or ticking sound that varies with engine speed and load.
* Heat shield rattling: A loose or damaged heat shield can make a metallic knocking sound, especially when the engine is hot.
* Timing chain slap: A worn or loose timing chain can cause a knocking sound, particularly at low engine speeds, but this is usually more of a clattering than a sharp knock.
* Valvetrain issue: A sticking or bent pushrod, or a problem with the lifter, can produce a knock that might be intermittent.
* Piston slap (cold engine): This is a common problem on some engines, especially when cold. The piston isn't fully expanded until the engine warms up, leading to a knocking sound that decreases with temperature. If it's only cold, this might fit your description.
* Something external: It's even possible the noise is coming from something *not* directly related to the engine, like a loose component in the accessories or even something on the car's chassis.
Why rods "don't just stop": A connecting rod bearing failure typically results in catastrophic damage to the engine. The bearing surfaces wear down, causing increased friction and generating heat. This leads to progressively louder knocking, eventually resulting in a seized engine. The sound doesn't just disappear.
What to do:
1. Don't drive the car: Continuing to drive it could cause major engine damage if there is something wrong internally.
2. Thorough inspection: Have a mechanic inspect the engine, looking at the above possibilities. Visual inspection may reveal loose parts or obvious problems.
3. Listen carefully: Try to pinpoint the location of the sound. Is it coming from the top end, bottom end, or somewhere else? Does the sound change with engine speed, load, or temperature? This will help diagnose the problem.
In short, while the sound might *seem* like rod knock, the fact that it stops makes a rod bearing failure extremely unlikely. Investigate the other possibilities first, but err on the side of caution and avoid driving the vehicle until it's been professionally inspected.
