So for a while now I've had a 5.1 home theater setup consisting of 2 Polk Monitor 60 series 2 floorstanding speakers, a pair of Polk Monitor 40 series 2 bookshelf speakers, a Polk CS1 series 2 center speaker, a Polk PSW505 sub, and a Yamaha RX-v3777 receiver, all connected with Coboc Model SPW-2C16-100-CL speaker wire. Before, there was nothing wrong with the setup; I would watch movies with them through HDMI, I would play music through my phone or laptop through them with the AUX and other daily things, but recently when I returned from college, whenever I turn on the sub alone, it makes a loud humming noise, comparable to a small engine, like a small motorboat running, but a lot quieter. Also the sub is connected to the receiver with a Mediabridge RCA to RCA cable.
Does the sub make the same noise when it's not connected to the receiver?
If it does then the built in sub amp is oscillating. I would contact Polk. They usually replace the amp. If the sub is out of warranty they will charge you but as I remember the price was more than fair. Not hard to replace.