I had a leaking gas tank and when i got it out i could hear something moving around inside i have not seen it yet but my tank repair guy said it was a spring from something.What could this be?

Could be off the end of the gas nozzle, or maybe someone put it in there, being a wise guy.

More than likely, it's an anti-siphon device that a previous owner installed in the filler neck. Lot's of gas theft going on back in the 70's, when the price went up.

My 69 had a spring in the tank also. 99% sure it was from the gas pump nozzle. Look at it as a bonus for paying $2.50 a gallon.

The anti-siphon springs were tapered. There were about the diameter of the filler neck at the top and maybe an inch in diameter at the bottom. Gas nozzle springs are 1" or so in diameter the whole length.