Rear car speakers sound shaky?

[Lazsher]

My parents just recently bought me a 2004 Pontiac Sunfire. Ive only had it for 2 weeks now. I like my music really loud, with a clean, hard bass. Today i realized that when a lot of bass comes through my rear speakers, it sounds just fine but at the same time sounds shaky, and seems to drag on. My speakers aren't blown because there is still music coming from them...... Im a 17 year old girl and have no clue what to do. So if you could please tell me what could be the cause and how do i fix it, that would be wonderful.

Thank You )

[QTIP]

Your rear speakers are blown. Just because you hear music doesnt mean they are not blown. Replace them and I guarantee they wont sound "shaky" .
Hope this helped.