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If it is in the top left of the engine, it is the vacuum pump, similar to a diesel engine vacuum pump. Due to the nature of the Valvematic engine with the throttle wide open, there is no vacuum created for the brake servo, hence the vacuum pump. Being a petrol engine which is more quiet than a diesel, it is more noticeable. It you take it back to Toyota, they may say the same thing. Notice the rattle does not get louder as the engine revs, as a cam/tappet does.

MANN is a well known brand. Damage due to the "wrong" oil is highly unlikely, especially in the short term.

Its JUST possible that a thicker oil is causing a delay in hydraulic tappet or variable valve timing actuation, leading to a temporary increase in valve train noise. Its also possible that a thinner oil is providing less cushioning and sound deadening, or that newer oil has cleaned some sound-deadening deposits, though this last would take a while and is probably unlikely.

Or its coincidence (which would be my best guess)

You have no possible recourse. The first oil change is gone. It would require sophisticated oil analysis to identify the second oil change, and it would be impossible to prove that any (itself unproven) damage was due to the "wrong" oil.

If this is a 287000 mile car you should congratulate yourself, it, and anyone involved in its maintenance. Buy them a beer, and going forward put the thickest oil in it that the maker allows.

I used to have something similar with a very high mileage old Passat diesel that had noisy tappets when cold. An engine flush (Wynn's I seem to remember) used before changing the oil helped a bit but could never make up for the well north of 200k miles.

The other noise it could be is a bit of piston slap whilst cold - again can be common in some designs of engine as the miles advance. Usually not a major issue.

At 287k miles - keep calm and carry on!

Using the wrong oil woun't show issues up straight away and MANN are very reputable filters.