strange rattle/gurgle/cough

My 1998.5 A4 2.8 Quattro car is exhibiting a new sound that I can't really describe. It doesn't happen consistently: it's a strange little rattle that ONLY happens when I'm engaging the car in first
gear from a dead stop and am doing so very slowly and gently, seems more prominent when the car is cold. Seems to do it while I'm engaging the clutch and immediately after first gear is fully engaged. The car has gone 95k miles, so what could be creating this noise? Could it be an exhaust issue? The car drives great, no loss of power, no weird gear shifts or clutch problems.
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