2001 Chevy Impala Knocking noise

Other Category problem
2001 Chevy Impala 6 cyl Front Wheel Drive Automatic 80 000 miles

Well My 2001 Impala is doing a weird knocking noise when I'm braking and I accelarating (a variation of speed). This is not a suspension and a direction probleme I went on some bumps and there was no ''knocking'' noise, when I turn the stearing left and rigth in accelaration I hear the noise but this is only because I'm accelerating, not because I turn the stearing. When I am in a constant speed there's no noise and If I roll at 60mph and I go up to 80mph I'll not hear that ''knocking'' noise. Normaly I hear the noise in slow speed! (First sorry for my english this is not perfect I know) and I would like to know what is the problem and if it's a urgent thing to fix or if I can roll safely for a few weeks (until the time I got enough money to fix it) Thank you very much!