1998 gmc jimmy is knocking
There are several things that could cause a knock, ranging from a worn crankshaft bearing(s) to a worn water pump.
You can further pinpoint this knock by defining if it occurs occasionally, constantly, just at a specific RPM, or only under a load.
What kind of a knock is it? Hi-pitched or low-pitched?
A more detailed problem description would yield you a more specific/helpful answer.
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