So today, I started my ZHP and put it in reverse to head home. When I started backing up, I encountered a strange phenomenon that I'm not even sure how to explain properly. From a stop, my car made this dreadful "thud" sound--while it was stopped it did not make the noise, but as I reversed, it increased in frequency-- *thud.....thud.....thud..thud..thud thud thudthudthdthdddd d d d ddddddddd*. The sound, as best as I can describe, was the same dull thud of bass-heavy music you might hear from a car stopped next to you at a light. It also seemed to be coming from my subs. It was a very low-frequency (pitched) note--probably less than 100hz. It literally sounded like a pure "tone", NOT a mechanical thump at all. It was quite loud, in the sense that I more "felt" it than heard it. It did the same thing driving forward. I stopped with the car running and gave it a look over, underneath and in the trunk. Nothing seemed out of the ordinary. The radio was not on (it rarely is in my car). When I drove off again, it no longer made the sound..or so I thought. As I came to a stop again I could hear the same "note", but very quietly. Am I crazy? Is it coming from something else?