To me it looks more like White-tailed Tyrannulet. The belly does show some yellow wash, but it is not really vivid yellow. The superciliary is not really strong in front of the eye, the wing bars are quite tawny/buffy toned. All that makes it look more like White-tailed, at least to my eye. White-tailed is also almost always somewhat to much more common than Sulphur-bellied, though both occur at San Isidro/Yanayacu. Much easier to do these guys on voice