I have the ER RC-180 that has the GX-4100 on it for the re-circulating aspect. OMG the noise. Nothing would make it quiet. And like you, I couldn't figure out what was making the noise.... internal I guess... i don't know. You know what we did with it.... threw a bloody big towel over it. The weight made the pump vibrate less and the towel muffled what was remaining. Such a shame. It's such an awesome
skimmer.
