Not the noise you want to hear when sitting down in a comfy chair. It will be accompanied by your bottom feeling a little less supported.
Looking under the chair, the first stage it to remove the hessian covering. You'll probably spot bits of zig-zaggy spring poking through this. Inside there will be several springs running parallel to each other.
Replacing a spring isn't difficult. First find an upholstery supplier in the local area. Re-upholstering furniture is a popular college course so if you are struggling, ring then and ask where they get stuff from. If all else fails, there is the Internet but your bum might prefer you don't have to wait for delivery.
Taking out the old spring involves knocking it free from the retaining tube at each end. A small hammer will do this as they are only held by their own springiness. You might even be able to pull the thing out with your bare hands.
If this fails, try grasping the spring with some big pliers and tapping these with the hammer. Sometimes the problem isn't brute force but getting your whacker in to the target.
You may also need to remove a retaining clip in the middle of the spring.
Next, compare the new spring with the old. The new version will be a standard length.
Cut the new spring carefully. I used bolt cutters as the metal is stiff but assuming you don't have any to hand, a junior hacksaw should do the job.
Clean up the ends of the metal.
Slide the new spring into the tubes at each ends of the seat.
Finally, in the chair I did, the spring was clipped to a wire running at 90 degrees across the base.
Replacement clips (Z-clips) can be bought but I found them difficult to use and just re-bent the old clip which I had removed earlier.
Finally, use a staple gun to re-fix the hessian to make a nice neat job.
All this took less than 15 minutes once the replacement parts had been bought. Total cost, including 3 spare Z-clips was £2.60.
That's a whole lot less than replacing the entire chair and far more satisfying than chucking an otherwise perfectly good piece of furniture away.