You may have a flex disc coming apart on you. Use a flash light and raise the driverside using safty jack stands as well. Look forward of the driveshaft where the driveshaft connects to the tranny, you will see a disc.
Look for the obvious cracks or chunks missing from the disc. Check carefully in the area of the bolts for cracks and/or tears.
There is alot of info here doing a search on flex disc repair if you are a DIY kind of person.

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