Unfortunately, whatever you do will cost some money, unless you can convince another friend to store stuff in your garage.
There are a number of types of acoustic foam on the market, which are sold as panels that adhere to the walls and will reduce the reverberation and brightness that you currently hear. If you’re handy, you can also build recording-studio quality sound baffles to surround your drums. I found a pretty good how-to article about building them at http://www.recordingeq.com/EQ/req0801/baffle.htm. I’ve also seen some people hang heavy drapes to deaden a loud room. Good luck!