I've no experience with the C4 or the Penguins, but I do think Aquaclears are very quiet: I've used them for years and have had almost no complaint about noise. I will say the motor can be a little noisy after maintenance (or when new) until it settles in over a couple of days. Also, the lids can produce an annoying buzz from vibration if they're not seated right. Just play around with it a bit though, and it goes away.
FWIW, I think they're great filters.
2x20-longs, one heavily planted. Platy rancher. They. Won't. Stop. Breeding.