Does a PFO (patent foramen ovale) ever cause a heart murmur?

NO. A pfo is a small communication between the two upper chambers of the heart and usually it does not cause a murmur. Now if the hole is bigger it may be an atrial septal defect (ASD) which would cause a murmur. So unless it's an ASD and erroneously called a pfo, it should not cause a murmur.