I had the same problem..its not likely, but i just bought the house....
Find the beam you want to hang the bag from, and the one on either side...if i am counting correctly, u should have 3 beams, now
Measure the distance between the beams, then drill 9 holes, three on each beam, forming a square, the centre hole being the one you would have hung the bag from if you were to hang it from only one hanger. (if u want, you can cross brace these.)
Hang the bag from the centre hole with a heavy spring, and heavy bungees from the outlying 8 holes to loops around the top of the bag (u may have to modify ur bag a little), or on to the bag hanger-loop itself (although this is less effective).
The bag should now hang evenly between all the bungees, with most of the weight on the top-centre spring...thus spreading the weight (and forces from getting knocked about) across 3 trusses.
It'll take some tweaking to get it right, but i've successfully hung a 55 kilo (120 lb) bag in an apartment using this method, and u couldnt hear it getting wacked from the apartment above.
Cheers
Bao