Depends on what is causing the noise. Loose thrust washers will rattle and knock for a VERY long time, not much to worry about. But loose gears (the nut backed off and it is coming off the cam) or separated gears (the center metal section comes loose from the fiber outer section) are not going to last long at all. You could pull off the timing cover and take a peek, or just keep driving it. No real damage takes place when a timing gear lets go, and it isn't much more work to replace a gear than it is to take a quick peek at one (although you have conrtol of when you take a peek!).

Well - I take that back - when I had a gear come loose once it allowed the cam to slide backwards until the fornt bearing was rubbing on the #1 lifter. Which ruined the lifter and the lobe, neccesitating a pull of the head to replace lifters and cam along with the gear.
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