This is wat i do, let start from the easy way first then we go to the hard way if problem still there. Check any miss fire on the spark plug lead, have u been clean/wash ur engine lately? it might cause the moiture build up in the spark plug champer tat can cause miss fire. Replace the spark plug if needed, Changing from 91 octane to 98 octane fuel can cause engine pinging when accelarate as well, if u did then u need engine tuneup to reset into high octane type. Replace fuel filter, check air intake filter. check alternator charging rate, it should b 14v when engine running and 12v when engine off. It should b fix after u do all this check, if problem still occur then i suggest u bring to ur local toyota dealer for u to check for u. Personaly i only take this option last, when i out of idea n have no choice to bring my car to the dealer. Why? they cost too much, the technician there just standard, they all follow the book and guessing most of the time, tell u to change this n tat end up u pay big money just only one little problem. Good luck n keep update see how it go. Cheers