I had the same problem with my sons 306 . Tried for ages and replaced the arm a couple of times to no avail . I ended up getting a replacement blade , one of those super floppy ones like they use on the later models and fitted it to the existing arm . Got it from super cheap . Fixed the problem instantly and stayed fixed . Should have done it a long time ago and they last so much longer . They are more expensive though but well worth it
Same, replaced the arm and then after that failed took apart the whole wiper mechanism to look for anything loose. My hunch is that its wear in the bearings for the windscreen wiper mounts. But if anyone knows for sure I'd be interested to hear it.