Ok thanks a whole lot, I think it's one of my custom-processed 808 kick indeed..
After soloing a few tracks, it was the one sound adding the most energy in that frequency area. It might even be a "mix" of what all of you guys pointed out actually. i guess I am starting to feel the limits of my very poor monitoring setup as well.
By the way, I find it funny that for a very narrow frequency band, close to a single frequency, my dB meter won't tell me it's peaking.
Anyway, thanks again, very much appreciated !