Timing chain

I'm starting to have wonders about my chain. 1983 mercury grandmarquis, 5.0. The vehicle has about 140,000 miles, and the timing chain was
replaced at approxamtly 70,000, although I've been told that it should last to 150,000. Do they ever (or even in rare cases) give any sounds of warning before breaking? I can hear a sound when the engine is in a certain RPM range (sorry, haven't used a tach yet). I can usually hear the sound on a moderate accelleration, and when the engine reaches a speed just before the transmission shifts from 1st to 2nd, probably at arround 20-25 mph (just trying to give an idea of the engine speed; if I put it in neutral/park and rev it to the same approxomate rpm, I still hear the sound). To describe the sound it's self, it sounds like a distant jackahmmer (pnumatic drill) busting into concrete. I say distant because it's not very loud, I can barely hear it with the windows down, but it is a very rapid metalic sound, like that of a jackhammer.
Before I go ripping the front cover off, I would appreciate some opinions on what this sound might be. If no one understands the sound, I can try and record an audio sample of it, if needed.
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