2001 Isuzu Trooper is this a big red flag?

I went looking at a car dealership _used_ and came across this 2001 suzuki trooper that I like....but when I test drove it it made this jerk-knock knock? While I was driving . It did that every now and then mostly when I would excellerate ... I am worries that its a bad motor- I am gonna do a trade in value for my 2001 ford taurus which runs like a clock-what is your opinion? Also their mechanic is going to check on that and fix it....also why would it jerk-knock? Thx. Btw it feels like its jerking from underneath the center of the suv? It also has power-drive.