Hope someone friendly out there can help me with my beloved Rocket Giotto that has started misbehaving; whenever I brew and the pressure builds up it has started making a "screaming" noise and while it is brewing, most of the water goes to the drain tray instead of in the cup.

So the machine uses lots of water and also makes a horrible and high noise. Only current benefit is that the sound does wake you up more effectively than the caffeine :-O

Sounds like some sort of harmonic coming from the pressure relief/pressure regulator (whatever they call it in that machine) bypass valve opening. The clue being that water is going into the drip tray. I would remove the adjustment screw, spring and piston from the valve, clean the bore, check for corrosion or foreign matter, etc., apply a light coat of silicone lube, and reassemble and adjust.

I agree with Randy's diagnosis - probably the OPV. Cleaning it by hand may well be necessary.

Do you descale your machine yourself? If so, putting a blank in the portafilter and running some descale solution through the OPV will prevent it from screaming and sticking again. You might try this first to see if you can get away without disassembling it.

I think it's more likely a too-high pump pressure adjustment

FYI, #13 in this pic is your expansion valve (aka OPV):


On rotary pump machines adjusted down to 9 bar or less at the pump, this valve normally stays closed while brewing. It opens and lets a little water drip out when all brew valves are closed and the water in the HX heats up and expands. On a vibe pump machine it functions as a pressure regulator/OPV, opening at 9-12 bar depending on how the valve is adjusted. They sometimes screech, but should not in your case, so check if:

- the rotary pump is adjusted too high
- the expansion valve was adjusted too low
- the expansion valve is somehow defective, failing to seal due to scale, bad seal, or bad spring


And your most likely cause is the first one. See this video: