When a water heater has a lot of lime in it from hard water, it will make a pretty loud rumbling, banging noise. That may be your problem. Draining/flushing probably wont get much sediment out of the heater when it is that bad. You can try to clean the heater (a big job), or replace it, or just live with it like the previous owner must have been doing. I personally dont like to see water heaters in attics because they are so hard to replace and repair. Maybe when you replace it, you can put it somewhere else.