Idlie problems 97'

Hello all, I just brought a 97' explorer(a week ago) I noticed after a short time running(1 and a half hour) it would start vibrating when at a traffic ligh). Is this a common problem with explorers. and what s the price for fixing it. I dont know if my warranty covers it. I want to know how serios this PROBLEM is, thank you all!

Nfixit