I put a similar comment in the old forum and will repeat it here in case someone gets a more definitive answer from Toyota.
I purchased my 07 w/30K miles, and the morning after taking it home I noticed the knock as soon as I started a light acceleration from 10-20 mph. Bummer! Screwed up engine. It sounded like piston skirt or rod bearing knock because it was only under load with the engine cold and cleared up as it warmed up. After about 5 minutes or 5 miles, it was not noticeable.... again matching piston slap or rod knock. While it could be something related to an exhaust leak, the sound did not match what would be expected.
It definitely was not ping or knock caused by fuel detonation,(that's hard to accomplish with a cold engine and light load anyway) and would not be fixable via a recall that modified mixture or timing changes. It also had nothing to do with the lifters as this would not be load dependent and lifter noise is a "lighter" sound. (engine talk)
So before going to the dealer service folks, I got the salesman to let me start a bunch of other vans and Camry's in the used lot with the same engine and mileage between 10 and 30K. I went through about 5 or 6 vehicles, starting each one, letting it idle for about a minute, then while holding the brake, put it in drive, and applied some gas. Sure enough, every one had the same crappy noise, and based on the repetition rate, it sounded like a single cylinder issue. Of course the salesman acted like he didn't hear anything abnormal, and honestly may not have, since he did not seem too mechanically knowledgeable. (I had to explain how a timing chain was different than a timing belt)
So it was good that my vehicle was not necessarily abused causing the noise, but I found it strange that all the engines did it. I did not test any brand new vehicles, and would be interested in how they compared. It would seem like an easy problem for Toyota to verify and at least give us some blah, blah, blah language stating it is considered "normal" and would be covered should it ever necessitate being repaired. I doubt it would ever have to be repaired as I have been around vehicles with this type of problem over the years, and while it might get worse over time, it is unlikely to cause a catastrophic engine failure. It just sounds like crap. It does keep your foot off the gas while the engine is warming though, which is a good thing.
Besides taking it easy while it warms up I also use synthetic oils to minimize wear. (Maybe not provable, but psychology comes into play too.) I have had the van for about a year now, and the noise has not become any worse at a given outside temperature. It did get worse as the temperature dropped during the winter, necessitating a longer period of easy driving. Also, as the temperature dropped, the noise could also be heard with the engine idling, not just under load. Assuming it is a dimensional clearance problem, this also makes sense since the slop would be greater at 25 deg vs 70 deg overnight temperature.
Bottom line: I'm pretty sure it is a mechanical clearance issue which I will have to endure for the life of the vehicle. However, it would be nice to find out that there is some external component known to make noise during the warm up cycle until it is no longer needed. It would be easier to accept the "clunker" sound knowing it was a peripheral device and not part of the engine structure. (psychology again)
One thing for sure, if I purchase another Toyota, I'll be sure to test drive it cold.