Dampen noise from relay
Buy a quieter one?

In all seriousness though a contactor/relay is always going to have some noise, that being said, unless it is a closed panel and/or the 50A circuit constantly draws the full 50A  you should be able to find a fairly reasonable solution in the way of an SSR, there are companies that sell them as assemblies with a heat sink. A friend of mine works for one that offers single phase AC SSR assemblies capable of switching 50/60/70/80/100A depending on the assembly, and each is rated at 100% duty cycle up to 40C ambient temp.