2008 Prius "Tappet" noise

One of my daughters is giving the other a 2008 Prius, which suddenly developed a noise that sounds like a loose tappet, maybe a bit louder and thunkier than a real tappet. It had been making a similar noise, but stopped before I got to hear it, then a week later, just as I was about to pass it on, started making this noise. At the suggestion of a skilled mechanic, I ran it for about 30 miles w/ Marvel Mystery Oil, then changed the oil & filter and added some Lucas Oil stabilizer- no change in the sound.

Then I took it to the mechanic who found all the lifter clearances were in spec. He checked compression (it was very good) and sequentially deactivated the igniters to check wrist pin and rod problems- didn’t hear anything there. It doesn’t sound like a loose timing chain (bad tensioner)- too regular, too clicky. Might be something in the VVT, but it’s hard to imagine how the VVT mechanism could make that kind of sound. Another skilled mechanic suggested a piece of carbon on the top of a piston, but agreed that’s unlikely in a modern engine in apparently good condition.

So, I’m baffled, my mechanic friend is baffled and my paid mechanic is baffled. The paid mechanic says, “Bob, you can’t be the only person with this problem- I’ll start quizzing all my colleagues.” Can anyone in the Car Talk community add to or subtract from the confusion?

PS- the paid mechanic doubts it’s anything in the CVT.