Hey y'all. I bought the BDB second-hand and it's making a rattling sound. I took off the cups as well as the drip tray and it continues to make the same sound. Last week it was not making this sound. Today I noticed debris in the water so I removed the reservoir, cleaned it, and refilled it with remineralized distilled water.

Any suggestions? Does it sound like it's struggling?

Check your pressure with the cleaning disc installed. Should be 10-11 bar if it has not been adjusted from factory setting.
I'd pop the top and inspect for leaks in the steam circuit and brew path. I don't hear anything terrible, but it's hard to tell from streaming audio sometimes.

I've heard leaks/cracks/loose fittings in the brew path cause strange gurgling noises and low brew pressure.

The leak check is just good practice with a used machine. I'd also pull the 3 way solenoid or open the steam boiler and inspect for scale.