Well, it didn't stay quiet long. I got my other 2 powerheads in today, and they are silent. So it looks the first one is defective. Instead of going through trying to send it back, I'm just going to keep it as a spare. I think 2 will do just fine for what I need.
I also tried hooking up the air inlet to one of them, and it just sprays out water. Lucky it didn't get my power strip below. I just can't figure out why it's not sucking the air in. Tried just about every possible thing. Ah well, can't win everything.