Catholics have always believed this proceeding comes from both the Father and the Son. The Filioque was added to the creed to clarify the teaching on the Trinity - AND it is quite scriptural too! See:
John 16:7 - But I tell you the truth: it is expedient to you that I go: for if I go not, the Paraclete will not come to you; but if I go, I will send him to you.The Orthodox cannot declare us heretics for following not only the TRUTH, but that Truth as it is explicitly expressed in Scripture! Clearly here Scripture attests to the fact that the Holy Ghost would be "sent" by the Son too.