If leaving out the flow restrictor 'solved' the problem, I'd start there. Maybe the pump is overdriving the flow meter/restrictor and the restrictor has become a whistle. Could be someone changed it out and installed one of an improper size.

erics wrote:Sounds like a good impression but let's change it to a "fact".

Now it's a fact!
Based on sticking my ear up to various components, my impression (again) is that the sound is coming from the boiler/OPV area to the rear of the machine, not the group. Also, if I leave it sitting unused for a little while, there is no flow from the group for a few seconds after the pump starts, but it behaves normally on subsequent water draws (unfortunately the video does not demonstrate this). At the moment, it's behaving particularly badly after all the tinkering with parts today.

Running the machine from an unpressurized reservoir was apparently making the problem even worse, since there is no check valve on the inlet and it was quickly leaking about 65 ml of water back into the jug. I re-connected it to the regulated pump and accumulator, and it's still grinding and squealing away almost every time I draw water from the group. I don't use a check valve with this setup, but the accumulator pump has its own check valve, and I have no issues with initial flow from the group.

I've tried adjusting the OPV and pump pressure to various settings, and I even tried removing the restrictor from the flow meter again. The machine actually still makes the same noise in this configuration, but it's just too quiet to hear without holding my ear up to it. I carefully checked the restrictor with wire when the flow meter was taken apart yesterday, and the group has an appropriate water flow with it in place (~650 ml/min). I've also tried fiddling with and damping all the various pieces of copper tubing. I'm open to any suggestions on how to proceed.

You could try an old mechanics trick of holding a wooden dowel against a component and also your ear or ear area. A metal rod would work too. I'm guessing it might be much easier to pinpoint the source if the noise was dramatically amplified, which it should become with this trick.
With a few seconds of poking around while the noise is taking place, you should be able to narrow it down to 'this component'.
Sometimes the 'why' of things is less important than the 'what' of things, esp. since you can change the 'whats' fairly easily.

This is a great trick when working with motors etc...

Would never attempt when poking things around that could slip and cause a connection to MAINS power...

Chris -

You are aware of this? - http://s1cafe.com/

The fact that it takes several seconds for water to appear after the machine has sat idle "for a while" indicates a failed check valve in the feed line to the brew boiler or a leaking brew solenoid valve. Water flow should be essentially instantaneous.

I'm aware of S1 Cafe, and I've tried posting about this a couple times, but the readership is very low these days, and there are so many sub-forums it is very difficult to get useful replies. There is a user over there who seemingly had an identical issue with his Vivaldi, which you can read about here. He even got a stethoscope to pinpoint the location of the noise, which he thought was coming from the restrictor after the flow meter. Unfortunately, the thread ends with no resolution, and he hasn't been online for over a year.

In my latest post, I mentioned that the initial flow problem only happened when the machine was running off an unpressurized jug, and it was caused by exactly what suggest (no check valve). I don't have this problem when the machine is connected to the usual pump and accumulator system. Probably a red herring?

To follow up, I've been in contact with several Vivaldi owners who experience this noise on a regular basis to varying degrees. It's also possible to hear it in some Youtube videos of the machine pulling shots, like this one:
It's probably not obvious to a lot of people, but there is quite a difference between the "squeal" and the normal whooshing sound of water passing through the flow restrictor as the pressure ramps up.

A potential explanation that was suggested is tiny amounts of air getting into the water line through the three-way valve or by other means. I'm not really sure how this would work in practice; it's pretty clear from the video I posted with the machine running from a bucket without an inlet check valve that any air in the line produces exactly the same effect. I've also realized that things like how much water I draw through the group, how often my external pump pressurizes the accumulator tank, whether the OPV has the typical Vivaldi drip, and so on make a big difference. If I manually pressurize the tank once a day and pull warming flushes against a blank basket, I can go days without hearing a peep from the restrictor. I'd actually switched to a flushing routine where I drew more water from the group right before I originally posted the thread.

A fat soda straw and a piece flexible hose will probably make a good enough stethoscope to let you get a decent idea of where the noise is coming from.

Ira

Bob_McBob wrote:To follow up, I've been in contact with several Vivaldi owners who experience this noise on a regular basis to varying degrees..........

You have a black cloud hanging over you when it comes to owning espresso machines. It truly sucks. Hope you get the GS3 replaced or serviced to your satisfaction too.