Hi Pria,
we have been having very similar issue, not exactly the same, but similar. We have a rental townhouse so there's nobody above or below our tenants, the issue for us is our next door neighbor.
She also complains, bang on the wall, complain about noise, she even use a device to record noise! and also have set up video camera pointing to the driveway.
she was either unemployed, or she's doing home business for some sort, that's why she's always at home.
When this first happened, we try to resolve issues by working between her, HOA and the tenants. However, after couple times we kind of knew what's up!
Long story short, i think our neighbor is just looking for action, seems like she's bored, or she's just not fix for apartment/townhouse/condo environment. One of our tenants sent her a warning letter from their lawyer, and she kept quite for a while.
we have also told her to call the police couple times, surely enough she never called.
I think telling her to call police actually help the situation becasue her complains were based on no basis, and she probably don't want to get in trouble for calling police because of silly reasons.
Our HOA has helped a bit too, a lot of time conflicts go directly to HOA, which save us some crazy phone calls and emails.
for you, how was the noise level between the upper and lower level? understand that certain level of noise do happen in these housing environment, but if they can hear every single thing that's happening, that could be a real issue.
are they crazy people being unseasonable? is it only the upper tenant that's causing trouble? or both?
Maybe investing in some sound proof ceiling/floor/wall? Or if they are being unresonable and just causing trouble, let them to deal with each other, or even tell them just call the police maybe the solution.