glinskonian wrote:Yes, here's the video of it running with the back flush disc in (although it makes the same noise without a portafilter).
video

The sound disappears when the machine reaches full pressure. I'm not sure if that is helpful in pinpointing the issue.

Thanks.

Sounds like it is cavitating. The sound goes away when water flow is zero, because the cavitation stops.

What is the condition of the inlet hose before the pump?

Hm that makes a lot of sense. The hose looks fine, I don't see any kinks.

I tried priming the pump with a syringe just now but that didn't seem to do anything.

Where does the water come from?

I have it running on the tank (I plan on plumbing it in eventually).

Is there a filter or water softener thing that could be clogging the inlet?

I'm not familiar with your machine, so this is just what I would attempt, if it were mine:

Can you separate the pump from the motor, and operate the motor? This would tell you if the motor is silent, or if it's the noise source.

And in the same fashion, can you power up the pump using a variable speed drill and clamping onto a shaft of the pump(if the pump has a shaft that sticks up and goes into the moto) I don't know how that pump is being powered, sorry.

Point being; if you can listen to them independently, you'll be able to hear who's the noisy one. Maybe the pump spins okay, but has some axial wear, and it's internals are able to "rattle" up and down, inside that housing, thus making the metallic chattering sound.

I ran the motor without the pump attached and it was silent. Sounded flawless to me. So it must be the pump.
I will try that sometime this week and see if that gives me any clues.

I did notice that the water line coming out of the pump was slightly bent in close to where it connects to the pump. I wonder is it possible that it was enough of a kink to affect the little jet thing that is at the end of the water line. Hard to really test that one without buying a new water line.

Maybe if you took some detailed photos of the water line(s), someone here could give you some advice?

Since plastic has a memory, you could probably remove the waterline in question, and get it hot enough with a hair dryer, that you could reshape it, and then run cold water over it to make it cool down and retain the shape you created.

Yes as mentioned before, it sounds like cavitation.
Do you use any resin filter in the sucking pipe?

Is there check valve presented?