What mounting scheme are you using for the tank? Is it wedged tightly over a large area into the chassis rails and secured by only one bolt? How’s it isolated from the chassis – exclusively pushing through rubber washers, or is it steel-to-steel-to-plastic? Is it on the floorpan, or vibrating next to it and hitting 10,000 times per lap? See if a couple pieces of Silly Putty between the tank sides and any metal it’s close to help.
What are you using for fuel/oil? E10 street gas, 100 oxygenated unleaded race gas, 100LL avgas, 98 or 110 leaded race gas shouldn’t damage a tank. If you’re leaving methanol in it overnight it’ll rapidly age an HDPE/HDPP plastic tank, same goes for copper-containing alloys of aluminum or magnesium. Hydrogen peroxide, nitromethane, and 1, 4 dioxin are hard on plastic containers and if you’re using any of them please stop before you poison someone or blast yourself to Jupiter the long way.