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The least expensive option is going to be to create white noise.
Otherwise, you'll need some type of sound insulation and dense pack cellulose might be able to be retrofitted into the wall cavity. But determining your level of performance expectation is the hard part. How perfect do you expect it to be? Is this a common wall to another unit, or all enclosed within yours? It makes a difference.
A tankless water heater can certainly be installed in a condominium - similar to any other structure. The easiest model is one that mounts externally - typically near the highest use of hot water or midway between multiple usage points. If there is some distance between the use locations, you can also install a D'mand pump to "call" the hot water so you are not wasting water waiting for the hot water to arrive. If you install a unit on the interior of your unit, e.g., in the previous water heater closet, the unit will require venting. And - FYI - one reason people love to install an on-demand is that you can actually repurpose a former water heater closet creating more useable space in your condo. One last piece of important information, you will need to check the size of your incoming water line. Typically water lines are 3/4 inches in diameter. On-demand units require incoming water lines of one inch. So you may need to upgrade your plumbing to accomodate the increased size of your incoming water lines. I hope that helps.
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