I would go over and knock on the door, wait about 10 seconds and open the door.Be careful but firm. 

What this doesn't account for is the amount of units that have been knocked down or the population growth and demand for new housing.With record high material pricing and labor pricing, I would be surprised to see a big uptick in new construction.  

Digging out a basement, knocking down walls, new roof, luxury finishes, new hvac, etc.

As them if you can make calls for them, write letters, door knock, etc for them all for free. 

Your not a loser or a failure if you get knocked down, we all get knocked down sooner or later.

However, if you are just cold calling or door knocking that sort of thing is more labor intensive vs fixed cost of the items above.

Water pipes freeze and have burst at least once, luckily they were homeoutlet under sink caught fire, again lucky they were homepower is knocked out regurally from weather elementsdownstairs rooms receive no heat from hvac system and is in the single digits without using a space heaterthe wall heating elements downstairs have been broken, replaced and broken again, and was told they cost 500-700 a month in electricity to keep the placed moderately warmthe list goes on and on, and they have notified the landlord of each occurrence.

Direct mail campaigns, setting up a website with the right SEO, door-knocking, cold-calling, text message and/or e-mail campaigns, etc.

Are there older photos you can review to see if its a stable condition or worsening.If it's placed concreted, it could've been a form blowout or perhaps they knocked the form at an angle. 

The plumber who installed the indirect hot water heater went over twice to address the banging, knocking, clanging pipes noise she claims to hear, and he told me he didn't hear anything and didn't want to deal with her anymore, as she was texting him daily and there was nothing more he could do.