A few years back we had an enconter that might have been caused by a mobilephone. While holding at pretty low altitude (3000GND or something) we noticed the vibration indication on one engine moving up to 5 without any actual vibration noticed from anyone of the crew. Captain made an announcement about checking for mobiles, one was found from a pax and switched off, after that the indication returned to normal (a tad below 1). All that on a 733. Never had that after or before although i am sure many more mobilephones had been on on other flights (including some on the flightdeck).