NV3550 here. mine sounds like a bunch of marbles bouncing around.
Noisy when idling in neutral, silent when pressing in the clutch pedal.
Seems mines sometimes a quiet box of marbles bouncing around and others pretty loud box of marbles as described above.
no other problems otherwise. fluids been clean when draining out. was red unknown fluid when I drained it the first time from PO, but I replaced it with penz synchromesh and didnt notice any immediate changes or long term changes in noise