The harmonic balancer can be bought at most McParts stores for less than $100 and for the mechanically inclined with a few basic tools it can be replace in less than an hour. Continuing to operate with the rattling harmonic balancer will eventually result in the pulley seperating from the balance weight and the belt dropping off.
Personally, I have pulled dozens of those pulleys off, welded the pulley to the fly weight and put the car back on the road charging 1 hour flat rate and never have had a failure… But if operated too long the wear will preclude this repair.
Good luck.