93 Ford Explorer...Transmisson humming

My 1993 manual Ford Explorer is making little humming sounds and they are the loudest when I shift into 5th gear. And when I let off the gas, the transmisson seems to jerk. I got under the car to see if any of the bolts were loose on the transmisson, but they I could not see any. The sound seems to becoming from the transmisson, but I need help on how to fix/find the problem. Or you can tell me what to do, or give me suggestions. Thanks!

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