I'll bow to your greater experience.

Don't remember such a dramatic change in tone on my old Gaggia Classic for example.

I will say that on shots where I was dialing in new beans, if my initial grind was substantially too coarse, then some coffee would emerge during the early stage, once the tone changed though it would gush out. This suggested to me that the pressure had ramped up at this point.

Again, these are only impressions gained from use of the particular machine.

I would think that's due to the fact that vibration pumps take a while to get up to full steam so to say...

My impression from pulling hundreds of shots on an Oscar is that I suspect the pump runs at a low pressure for an initial period, before kicking in fully.

This time period did tend to be variable, as seen by delayed pump kicking in NOT delayed emergence of brew.

If someone can explain how a tighter grind would cause the pump to kick in later on a normal vibe-pump system, then I'm all ears. My understanding, according to Dan's back-pressure statement would be that back pressure would be reached sooner on a denser puck, and the tone would change earlier.

Conversely on a loose puck, the noise would change in the 4s region, there would be slow emergence prior to this, and then it would gush out on the change of noise.

Apologies if my explanation is scattershot, I no longer own an Oscar, I have no reason to defend it, in fact I've been quite critical of it at times. I've owned an operated espresso machines with and without some manner of preinfusion, I'm familiar with a typical vibe ramp up, I'm familiar with an E61 preinfusion ramp, the Oscar is distinct although closer to the latter.

Of course NS could be making false claims in their advertising materials.

I'm all in for the Oscar. I just pointed out that it doesn't actually have any preinfusion. But it is fairly gentle on the puck and produces excellent espresso.

About the claim. I haven't seen Nuova Simonelli make any claim on the Oscar having any preinfusion. Their engineer was quick to point out to me that it didn't have any preinfusion. Seems to me that some resellers kind of over sold it. If I'm wrong please post a link so I can ask NS about it next time around

Dan is absolutely right in his explanation of both sources of noise. At 45 secs for the shot the machine is basically choked and is protesting. I'd guess not only was the grind too fine, but I'd bet the basket was very full. The brew water flows until the puck is saturated, at that stage the swollen grounds further restrict the flow. This is particularly noticeable if the puck is hard up against the screen. It can be heard as a change of pitch as the pump is at maximum pressure acting against the OPV.

To HB -There is no OPV on an Oscar though other than a safety valve which releases at 1.6 bar. My biggest concern with that video is that it took nearly 50 seconds to get a shot, something isn't right with that. Perhaps the noise is the pump struggling to work, as in the tamp is too hard or the grind too fine, or any combination of the two. I have an Oscar and get the standard 25 second-ish shot easily, and I do not flush for nearly as long either (notice how it drops to almost 50, that is a big swing in temps to recover from before a shot). You might consider adjusting the P-stat a bit too since you seem to be pulling your shots around anywhere from 83 starting but quickly getting to the 96-107 c (I'm assuming) range and that is close to boiling in Farenheit, look for around 92-93 C. I think leaving it on longer to stabilize the temps and adjusting the temperature via pressurestat to maintain that with a shorter cooling flush will get you far better results.

To HB -There is no OPV on an Oscar . . .

Sure there is:

Maybe your machine is missing this? This is a requirement to (a) limit/adjust the pressure that the coffee is exposed to during brewing & (b) limit the pressure that will build in the hx circuitry due to the inherent thermal expansion of the contained water during idle conditions.

The Oscar does not have any preinfusion beyond the period of time that the water starts flowing from the group and the puck creating full back pressure on the pump. If you want to call that preinfusion that's ok but then every machine with a pump has "preinfusion".