It could just be that its extremely similar... with a different note here and there, perhaps a different rhythm, or different progression. Any of these could cause that 'rings a bell' feeling without actually having been sampled. I find a lot of songs sound extremely similar, despite being originals. It does sound familiar to me.. but like you I cant pinpoint it... but yes, its quite a powerful...tip of my tongue, sort of feeling. If you figure it out, please let me know cuz its bugging the hell out of me now. lol THANKS JERK! LOL Just kidding! Good luck oh and of course I'll tell you if I figure it out too.
Jessy
xoxo