You may be catching the beginning of an IMS bearing failure. Many people who've had that happen describe the sound as a clattering, like if you filled a coffee can with heavy bolts and shook it violently. If this is what you're hearing, don't even think of starting that engine again.
If you can catch an IMS bearing failure early, the engine can be fixed. If the intermediate shaft breaks, the engine is almost always toast.
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Gator Bite
PCA Cayman Register Advocate
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