From what I found with my 1200, run internal, or basically as a powerhead, nice and quiet. Run external, or with any amount of PVC attached to the input or output - and it's a rotten little pump.
A few people have been by when my 20 was running, that darn pump rattled just about constantly. But in its current configuration in the 65 (basically a big powerhead) it's absolutely silent.
Andy
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