How do i add a noise (ex. endportal) to a block?

If it's custom, then you should enable block tick updates and open the "on block tick update" trigger. Then, make an RNG system to define how often you want it to make a sound. You should find a "play sound at X Y Z" block or something like that and insert the name of the sound you want to play. If it's a pre-existing vanilla block, make the same procedure with a "block tick update" trigger, but make an extra condition to go together with the RNG to check if the block is the same as the one you want to emit sound from.