Toyota Repair: engine acceleration, engine harness, serpentine belt

Question
Hi Autoly, we own a 1998 Sienna xle. Today it did something strange. While turning a corner the engine accelerated but only showed up on the tachometer, it went up to about 4000 rpm and then settled almost immediately back to normal. There was a loud squeal associated with this. The van itself did not accelerate. Could this be a slipping serpentine belt or transmission issue allowing the engine to rev without an increase in speed itself. Thanks

Answer
It's hard for me to give the answer now. I need more info, if you could offer.
1. gear in park, start the engine, turn steering wheel side to side, Will this duplicate the problem ?
2. Did someone service the engine harness or driverbility repair ?
3. when the problem happen, you apply the gas pedal or not?
4. how is the weather, heavy shower ? how is the road surface condition, wet and slippy ?
5. the problem happen all the time when you make a turn ?