Engine noise after 20+ miles of driving

[ray240sx]

Can someone tell me if this is the case with the infamous timing chain problem among all 240sx owners. I have a 91 240sx with 159,000 miles and the noise is coming from the front of the engine area. The noise happens when after driving the car for 20+ miles and not when i first start the car. It makes this knocking sound as if though metals were rubbing against one another. Its been like this for 3 months now. How much will it cost to fix the timing chain and how long can i drive without further engine damage? please help... thanks.