How can I mitigate low frequency reverb in a bedroom?
Read about bass traps.  There are all different types; you can build your own or buy one pre-made.  It sounds like for your situation, a correctly tuned Helmholtz resonator will accomplish the task in a small package.  It's supposed to be cheap and easy; try this calculator.

You can determine the frequency of your noise with this app or similar.  Make sure that you see a clear low frequency peak because your phone might have a high-pass filter.  If you don't see one, then try putting your phone against the wall or against the AC unit, or try another phone/tablet/etc.