Over the last week or two, my millenium has gotten progressively noisier, making a loud buzzing sound while pulling shots. I also noticed that when backflashing the machine, I am getting much less water out of the release valve (not sure if that's what to call it, but the tube that dumps pressure into the drip tray after pulling shots). Are these symptoms of my pump going out?

The machine seems to work fine otherwise, and I have not measured the pressure at the portafilter recently. Still getting yummy shots, but the noise has me concerned.

I found the articles on Elka Pump rebuild, and also OPV issues (sounds like the symptoms for that are different though).

how often and how well do you descale it?

I bought the machine used about 4 months ago and descaled it pretty thoroughly then. It was in pretty bad shape, I also thoroughly cleaned the group head and replaced the gasket & screen, fixed a few leaks, etc.

Filtered water only.
I backflush after nearly every session, water only, and backflush with Cafiza about every 4 weeks.
I pull 2-4 shots a day.

Maybe a descale might do the trick?

It could or couldn't solve the issue

is the buzzing happening only when the heating exchanger is flowing or also when the boiler is autofilling?

in the meantime
just for eliminating probabilities
check the inlet filter screen that is clean and has no obstructions ( same if you have a reservoir softener)

Thanks for the help Stefano.
The buzzing happens when the lever is pulled up (to pull the shot), and also when the boiler is filling.

I'll try a descale and also see if I can measure the pressure at the PF....

Pressure at the grouphead checked out at 130psi. I did a descale & thorough flush- seems like the sound is gone now, or at least the volume is back to normal. I think it was the handle/screw on the reservoir rattling against the interior of the casing.