Check the stabilizer bar bushings. Inexpensive, relatively easy to replace. If your struts are good, and those bushings are good, very well could be the upper strut mounts as you suspect. Apart from the swing arm bushings, which rarely fail, the front suspension is a fairly simple assembly - there's nothing else that would make that noise. Except when you say "rattle" - could be something else loose, like a plastic inner fender shroud. Strut mount noise will be most noticeable over a slightly bumpy surface, and you'll hear it just once over a larger bump. If you have a continuous rattle, it is something else. Once had an odd experience where a new factory original strut was noisy and immediately replaced under warranty.