10 bucks says it was when your fan turned on. You didn't notice it throughout the rest of the day because you were driving. When you came to a stop your fan probably came on and sent your idle down, thats why a few seconds later it popped back up...because the fan turned off. Just my gut feeling. Peaceout
i raise you $20
but seriously..that would be my guess as well. when the fan comes on, engine noise will increase some (not a lot) so if you were listening to music/radio/whatever..you most probably wouldnt have heard it, just felt it.