Tweeters Pop, No Amp

Hi everyone. I have a super-basic setup in my Xterra. I have a Clarion FTZ105BT deck, a Kicker 11HS8 Hideaway sub running at line level. Tweeters are Kicker 43CST204. All other speakers are stock for now. There is no amp driving my speakers other than what's in the deck. On my Clarion, around volume lvl 18, there starts to be some serious popping on the tweeters. These are brand new, so I don't think they're damaged. Plus the stock tweeters did it too. I've read that it could be the speakers bottoming out, but does the Clarion provide enough power to do that? I've also heard it could be amplifier clipping. I wouldn't even know where to begin there since I don't have an independent amplifier. Lastly, something I find strange, is the popping appears to be frequency dependent (since it's a pop, and not a buzz, and doesn't always pop at top volume.) The tweeters have your basic inline electrolytic capacitor high-pass filter. I believe it's around 3.5kHz. I have my Clarion EQ set to Low: +6, Mid: +2, and High, +3 (all out of 7). XBS and other modifiers are all turned off. Any ideas? (Please do not suggest I go out and buy $500 worth of new equipment. I have a 1 year old!!) Thanks