XC60 D5 - popping noise: vacuum pump check valve / solenoid?

[RubberArm]

First time posting on the forum, so from the start just want to say thanks for all the useful info and great folks here that I have benefitted from in the past!

On my XC60 D5 (MY 2012, engine D5244T11 / M66AWD) I have the 'popping' sound as reported in these threads, that go when the brake pedal is depressed a few times:

1. https://www.volvoforums.org.uk/showt...60+vacuum+pump

2. https://volvoforums.com/forum/volvo-...-2012-a-92028/

So I went ahead and replaced the external vacuum pump non-return valve (using Volvo original part 31430694 - a very expensive small piece of plastic as everyone notes!) hoping this would solve it. It unfortunately hasn't (of course, couldn't be that easy!), and the valve I took off was fine, membrane intact, etc.

I called my local Volvo garage and the guy there said that there is another valve that can fail which I think he means is a solenoid valve, which I think would be either:

1. part 31422233 (https://www.skandix.de/en/spare-part...valve/1062478/), or

2. part 31219138 (https://www.skandix.de/en/spare-part...ducer/1079468/)

Excuse my ignorance, but are there two different solenoid valves, and does it make sense it could be one of these causing the popping sound?

If so, where is/are they located? Perhaps I'm being dumb but I can't work out where...and what would be best next steps so I can avoid the dealership hourly rates.

Many thanks in advance for any assistance!