Toyota Repair: Toyota Sienna engine noise - normal?, engine gasket, dealer warranty

Question
Hello:

I own a 2001 Toyota Sienna CE.  I bought it second hand a couple of years ago, but as a certified pre-owned from a Toyota Dealer so it still has a little warranty left.  This was the first time I bought a used car under dealer warranty so I never took chances by taking it to non-Toyota service centers, even for oil changes, because I didn't want to risk losing my warranty rights.

Anyway, here's the problem:  There's some weird noise that we keep telling the servicemen about every time we go for service (since I bought it) but they don't seem to know (or indicate) what it could be.  It happens when going up hill, also around 30 mph and 70 mph (I don't have a tachometer to tell the RPMs).  Originally, I thought it was engine knocking, piston problems, and so forth, but the servicemen say that the engine's computer doesn't indicate so, etc.  When I thought more about the noise and how to describe it, it reminds me of how diesel engines sound -- well, not that loud, but I'm trying to describe the ringing/vibration-type of noise.  Again, this happens in certain situations (steep uphill, certain speed/RPM).

BTW, a few months ago the engine light came on and it was an problem code about some knock sensor.  The knock sensors (both) were replaced, at Toyota, as well as having replaced some engine gasket related to the work (as suggested).  I'm not saying that this problem was related to the noise but I thought it would have been.  But, it wasn't as the noise is still present!

Could you please suggest a possible cause and what I should point the servicemen to.  Sometimes I wonder if the Toyota servicemen are trying to avoid fixing a problem that's under warranty.

Thanks in advance.

Answer
Any engine noise like this is not normal, it sounds like the engine is pinging/knocking due to a condition that's known as detonation or preignition, both conditions are very harmful to the engine. This is why the knock sensor codes were set, not because the knock sensors were bad but because the sensors are designed to adjust the engine timing up to a point when pinging /knocking is detected, after the sensors exceed their operating parameters they can't do any more ignition timing adjustment and notify the computer which then turns the check engine light on. There is a serious problem with this engine, the dealer needs to find why this is happening, if necessary have one of the service techs or the shop foreman or service manager go with you on a road test and show them the noise and see if they still think it's normal, ask them if they thought it was normal and acceptable if it was their own car.