I have a 1999 Toyota Sienna van, 150,000 miles. Never had any mechanical problem in the drive train. (Replaced the battery once or twice, have several issues with electrical accessories.) The car has been amazingly trouble-free. Until now.
When I accelerate and the engine RPMs move through about 2300, I get a sound that sounds like the sound you hear just when a circular saw first makes contact with the wood, only it sounds more like metal-on-metal. Just for a moment. If I slow the acceleration and hang in that range longer, the sound lasts longer. I can get it to make the same sound if I rev the engine in park. It does it regardless of what gear it is in, always at about 2300 RPM.
It’s just a noise, but noise usually is a leading indicator of bigger trouble.
If it always happens at the same RPM level, that is probably because at that speed the engine is emitting a resonant frequency that is causing something already loose to vibrate, which gives you your metal-on-metal sound.
I’d bet it’s something as simple as a heat shield or perhaps some sort of skid plate.
Someone else might tell you your catalytic converter is coming apart. Drive 2 wheels up on the curb, get the car to make the noise and listen underneath. If the noise is outside the muffler or cat, tighten up whatever it is, but if it’s inside you can sometimes quiet them down for a while with a couple of big drywall screws run into the works.
I thought about the catalytic converter or the muffler for a quick second, but I’ve had both problems at one time or another and when they come apart you typically get weird motor performance due to back-pressure problems.
I have a 2000 Corolla with 140K miles with what sounds like the same thing. It tends to happen right before it shifts (when the RPMs are higher - no tach in my car to see the exact rpm). If I push it on the highway I can get it too. It sounds perfectly fine most of the time. Oil levels are ok. Transmission oil is ok.