When it started I was on the highway and thought sand was hitting the windshield. Been living with it for several weeks now and it's obviously coming from the speakers. Oddly it occurs whether or not the radio is even "on" but these days nothing is really off it's all software controlled. Dealer said to bring it in but all they'll do is the "Factory Data Reset" from the radio's menu, so I found that myself but all that did was forget my presets, the problem persists. Also I had updated the software some weeks prior - my guess it that may have fixed some issues but introduced this problem, but it's hard to tell. Dealer said they have heard of the issue but there's no fix yet and nothing to do but wait for Honda to issue a software update. Sigh.