Could be a couple things:
1. Dirty burners- this will cause an inconsistent fuel/air mixture
2. Bad ignitor- If ignitor is slow in acting an overage of gas will enter chamber before combustion can occur
3. Check intake/exhaust pipes for restrictions
Dirty burners can be cleaned with pipe brushes or small wire. Testing a faulty ignitor is usually done with an ohmmeter. I would recommend getting a combustion test which should include testing you inlet and manifold gas pressures. This loud boom on start up could cause your heat exchanger to crack and release carbon monoxide into your supply air to the building. A combustion test would determine if carbon monoxide is present in the supply air. Carbon monoxide detectors are always a good idea in the home as well. Hope this helps!