Crickets make an intermittent sound and should be easily differentiated from tinnitus. Cicadas, on the other hand, make a constant high pitched buzz that I find quite painful, even through a closed car window. But your test of blocking your ears to see if you get any relief is probably the most practical test. As far as hallucinations go, I can't see why such a meaningless noise can be brought up by our psyches into any real hallucination. I may be wrong, but I wouldn't get hung up on it being anything clinical to worry about. Chat to your doc by all means, but don't loose any sleep over it if you can manage to ignore it.