Engines can make many different noises and people will describe them as knocks, pings, rattles or thumps. A true "knock" is caused by rod or main bearings hitting against the crankshaft. The general cause of that is either too much space between the bearing and crankshaft or insufficient oil pressure to maintain the distance between the bearing and crankshaft. Sometimes you can use a heavier grade oil so that the oil doesn't leak out of the bearing space so quickly. If you're currently using 5 W 15, go to a 10 W 30 or maybe even a 15 W 50. If you can't make the noise go away with that, either the bearings are too worn or it's a different problem. If the engine has less than a couple hundred thousand miles, the only reason for bearings to go bad would be inadequate maintenance.
Other problems that can result in noises like that are:
Piston slap. That's when the skirt (lower part) of the piston is slightly worn and is enough smaller than the cylinder that the skirt "pops" from one side to the other as the engine runs. It's more of an annoyance than a real problem, especially since piston slap typically goes away as the engine warms up a little. It typically takes just a few seconds to go away. If it's piston slap don't worry about it until the engine is rebuilt. Piston slap is quite common among many of the GM 350's, especially in engines that go short distances a lot or don't get adequate maintenance.
Fuel "pinging". That's when the fuel ignites too quickly and causes a sound like something rattling around inside the engine when you step on the accellerator. Use a fuel octane booster or change the engine timing.
Hydraulic lifter noises sounds more like a tapping sound and is usually caused by inconsistent oil changes
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Where does the knock noise seem to come from? Is it affected by engine temperature, louder cold then hot?
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