Idling vibration.....

[flyromeo3]

.....My sister has a 01 accord sedan 4cyl and I notice when her car is at idle. (doesnt do it all the time ) I could feel/ hear a slight vibration coming from the passenger floor board.

I tried prying up the carpet to see if anything might of been dropped but there is nothing there. Its coming from around that area i just cant figure it out..

Any help will be appreciated.