I have been installing systems in my vehicles for over 10 years and have never had anything as strange as this happen...
I am running 4 8" Infinity Reference 860W subwoofers. Nothing about the setup changed except the new subs and new custom box. Amp is the same, deck is the same, wiring is the same.
Here is the bizarre happenings: When I'm driving down the road with my windows UP, the subs sound amazing! But as soon as I roll down my window even 1/4 of the way, the subs sound like they are distorting, making a sort of 'pop' noise. If I am parked and my window is down, they sound fine...
It also does this at low volume or high volume, so it's not an over-power to the sub problem.
So long story short, it seems to only happen when I am driving 30mph+ and have my window rolled down. The noise is definitely the subs, as it does the 'pop' on almost every note.
Figure it out, win a case of beer! Seriously! It's driving me NUTS