If you do find that it is rust in the tank you can do the old hotrodder trick-
Take out the tank, put a chain say 1 metre long inside the tank ( but first tie some rope or wire to the chain so you can get it back out!) then jump up and down and basically make plenty of noise with the tank so that you brake loose most of the rust. ( Just screaming and making noise doesn't work!)
Then flush the tank out and there is a fuel tank sealer you can buy, it's like a 2pack paint ( but fuel proof) Poor the stuff in and do another dance and then you will get another 10 years out of your tank!