It has gotten louder in the last couple of days.
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1999 Nissan Altima Question: Manual transmission makes noise when under load going forward. Not in neutral.
Answer #1patrick mannion from Greg Solow's Engine Room, October 20, 2011, 21:09Master
Check the transmission for oil or perhaps just change the oil as a precaution. If it makes noise in all gears and not in neutral or the clutch pedal depressed the bearings in the transmission may be bad requiring the transmission to be overhauled.